Giving the strict nature of so many Catholic teachings it would seem that their would be a lot of folks who find abiding by the rules just to difficult. Pre marital extramarital sex contraception use etc... Are there significant numbers who are excommunicated and who makes the decision to do so?
I do not think that any hard numbers are kept about excommunications, because excommunication is intended to help persons repent of their sins, not to pillory them in public.
However, the short answer is that there are very few excommunications imposed or declared each year.
Note that none of the sins mentioned by the O.P. merit excommunication. Excommunication is reserved only for extremely grave offenses that, in the judgment of the Church, cause grave and lasting harm to the faithful or to society, and hence require special treatment in Church law.
Some background regarding excommunication
Excommunication is a penalty imposed for extremely grave offenses that Canon Law specifically treats as delicts or crimes. When applying the penalty, the Church does not consider only the severity of the offense, but also whether, given the circumstances and times, certain goods need special protection under the law. (See Catechism of the Catholic Church [CCC] 1463.)
For example, because unborn children are extremely vulnerable nowadays to abortion, the Church punishes those who procure abortion with an automatic excommunication. (There is debate among canonists as to what “procuring” abortion consists in, but all agree that the doctor performing it and the medical staff strictly necessary for the procedure to occur fall under this penalty; see Code of Canon Law [CIC] 1398.)
There are also penalties of excommunication for various sacrilegious acts, such as desecrating the Eucharist, revealing the secret of Confession, assaulting the Pope, and so on. (There is actually a fairly complete list on Wikipedia, for the curious. It seems to omit the relatively new penalty for attempting to ordain women.)
The effects of excommunication
It is important to remember that excommunication does not remove a person’s membership in the Church. It merely imposes a penalty which, among other things, forbids that person from receiving the Sacraments. (It also, for example, prevents a person from holding an office in the Church.) Communion can be restored by repenting of the action accomplished, and seeking reconciliation from a priest (or bishop) with the necessary faculties to lift the excommunication. Also, any person may receive the Sacrament of Reconciliation when he is in danger of death. (In such a case, if he recovers, although his excommunication has been lifted, he is obliged to make recourse to the competent Church authority as soon as he can reasonably do so.)
See CIC 1331, for more information on the effects of this penalty.
Latae sententiae vs. ferendae sententiae
An exact answer to the O.P.’s question is complicated by the fact that the Canon Law contemplates two kinds of excommunication: one that is incurred simply by committing an act (latae sententiae); and the other, imposed by a competent Church authority (ferendae sententiae).
Latae sententiae excommunications are generally not known to the general public, and public records are not kept about them. Reconciliation with the Church is done entirely in the so-called “internal forum”—that is, secretly, in the confessional. Hence, it is extremely difficult to estimate the number.
(My personal estimate, as a priest, is that the number is relatively low, because a number of stringent conditions are necessary for the excommunication to occur: the person must be an adult, must be free from grave fear, and must be fully aware that excommunication results from his actions.)
Latae sententiae excommunications, in particularly grave and public circumstances, can be declared—as happened in the case of Archbishop Marcel Lefebvre and Bishop Antônio de Castro Mayer, and the four bishops consecrated by them, in 1988.
An excommunication can, on the other hand, be ferendae sententiae; that is, imposed as a penalty by competent Church authority.
Counting declared latae sententiae and all ferendae sententiae excommunications together, there is only a small number each year, especially considering the sheer size of the Church—some 1.3 billion according to CARA.
(There is, oddly enough, a list kept on Wikipedia of Catholics excommunicated by year.)
An excommunication is a particular type of Church penalty known as a censure. Censures are what are known as "medicinal penalties" (canon 1312 of the Code of Canon Law, section 1, note 1), meaning that their primary purpose is to help a person realize just how seriously wrong their actions have been, and encourage them to reconcile with the Church. For this reason, I'd be surprised if there were a formal list kept—it's not a "Ten Most Wanted" list. I'd expect the number to be pretty low, in the dozens or less (out of more than a billion Catholics) every year.
Per Canon 1331, excommunication prevents a person from receiving most sacraments, except in danger of death; from celebrating the Eucharist (if the offender is a priest); from exercising any ecclesiastical offices (for example, it might prevent a bishop from ordaining priests, or even from moving a priest from one parish to another); and from being granted any official positions within the Church.
The Church recognizes two types of excommunications: latae sententiae, which are incurred simply by doing an act, and ferendae sententiae, which are formally declared by an ordinary (usually a bishop), following an ecclesiastical trial. Most excommunications are intended to be ferendae sententiae, not affecting a person until the penalty is specifically declared. Canon law doesn't specifically state "the following actions are to be punished by a ferendae sententiae excommunication"; instead, it routinely uses the formula "X is to be punished with a just penalty", which is not necessarily an excommunication (reserved for the most serious cases).
Church leaders are specifically told to minimize the number of excommunications formally declared:
Penalties are to be established only insofar as they are truly necessary to provide more suitably for ecclesiastical discipline.
(Code of Canon Law Canon 1317)
A legislator is not to threaten latae sententiae penalties [which can include certain excommunications, as described above] except possibly for certain singularly malicious delicts which either can result in graver scandal or cannot be punished effectively by ferendae sententiae penalties; he is not, however, to establish censures, especially excommunication, except with the greatest moderation and only for graver delicts.
A penal precept is not to be issued unless the matter has been considered thoroughly and those things established in canons 1317 and 1318 about particular laws have been observed.
(Canon 1319, section 2)
Furthermore, there are certain cases in which excommunication simply does not apply: in which a person cannot be excommunicated. First of all, the offender has to have done the act in question deliberately, and "with malice aforethought" as it were. Without this, there can be no excommunication:
No one is punished unless the external violation of a law or precept, committed by the person, is gravely imputable by reason of malice or negligence.
(Canon 1321 section 1)
A whole bunch of groups of people are excluded from being excommunicable:
- a person who has not yet completed the sixteenth year of age;
- a person who without negligence was ignorant that he or she violated a law or precept; inadvertence and error are equivalent to ignorance;
- a person who acted due to physical force or a chance occurrence which the person could not foresee or, if foreseen, avoid;
- a person who acted coerced by grave fear, even if only relatively grave, or due to necessity or grave inconvenience unless the act is intrinsically evil or tends to the harm of souls;
- a person who acted with due moderation against an unjust aggressor for the sake of legitimate self defense or defense of another;
- a person who lacked the use of reason;
- a person who without negligence thought that one of the circumstances mentioned in [points] 4 or 5 was present.
So if you're a minor, or if you were (or felt) coerced into an action, or didn't know that what you were doing was forbidden by canon law (and I emphasize this because very often Catholic lay people aren't aware even of the existence of canon law), or if you were reacting to a situation over which you had no control, or if you thought you were behaving reasonably in self-defense, you cannot be excommunicated. Period.
The three specific instances you bring up—extramarital sex, premarital sex, and the use of contraception—are certainly grave sins: but they are not excommunicable offenses. Excommunicable offenses might be things like a priest (who is not a bishop) trying to ordain a priest, or a person physically assaulting their bishop or the Pope. The things you bring up are sinful, but they don't indicate (as these others do) a malice toward the structure and nature of the Church itself.