Roughly how many churches were built in each century?
This question is actually impossible to answer, even remotely. To put it bluntly: It is complicated!
To even try to estimate this, even roughly would prove to be not possible.
To start with we should consider that as Christianity advanced in it evangelizing of the people of the known world, it would need establish small churches in converted villages and towns and larger churches in bigger cities, with cathedrals (dioceses) being built in major cities. Each diocese had churches throughout the ecclesiastical region, in towns cities and even some remote villages.
Another thing that most people do not realize is that as Christian monks built monasteries across Europe, these in turn established many priories throughout the territory. Thus a monastery established in central Gaul could have dozens and dozens of priories that would be under the control of the main abbey and its’ abbot (in some circumstances, the closest cathedral). This enabled the monks to work in fields 30-40 kilometres away from one another. This also permitted travellers and pilgrims to travel from one place to another with a certain amount of safety.
Thus the whole countryside had religious buildings of one sort or another throughout the land.
St. Benedict of Nursia (480-543) were about visitors to monasteries in the Rule of St. Benedict, chapter 53:
On the Reception of Guests
Let all guests who arrive be received like Christ, for He is going to say, “I came as a guest, and you received Me.” And to all let due honor be shown, especially to the domestics of the faith and to pilgrims.
As soon as a guest is announced, therefore, let the Superior or the brethren meet him with all charitable service. And first of all let them pray together, and then exchange the kiss of peace. For the kiss of peace should not be offered until after the prayers have been said, on account of the devil’s deceptions.
In the salutation of all guests, whether arriving or departing, let all humility be shown. Let the head be bowed or the whole body prostrated on the ground in adoration of Christ, who indeed is received in their persons.
After the guests have been received and taken to prayer, let the Superior or someone appointed by him sit with them. Let the divine law be read before the guest for his edification, and then let all kindness be shown him. The Superior shall break his fast for the sake of a guest, unless it happens to be a principal fast day which may not be violated. The brethren, however, shall observe the customary fasts. Let the Abbot give the guests water for their hands; and let both Abbot and community wash the feet of all guests. After the washing of the feet let them say this verse: “We have received Your mercy, O God, in the midst of Your temple.”
In the reception of the poor and of pilgrims the greatest care and solicitude should be shown, because it is especially in them that Christ is received; for as far as the rich are concerned, the very fear which they inspire wins respect for them.
Another examples this, we see in St. Bernard of Montjoux establishing a canonry (Church) and hostel at the highest point of the pass, 8,000 feet above sea-level, in the year 1050, at the site (Great St. Bernard Pass and Little St. Bernard Pass) which has come to bear his name:
Since the most ancient times there has been a path across the Pennine Alps leading from the Aosta Valley to the Swiss canton of Valais. The traditional route of this pass is covered with perpetual snow from seven to eight feet deep, and drifts sometimes accumulate to the height of forty feet. Although the pass was extremely dangerous, especially in the springtime on account of avalanches, it was often used by French and German pilgrims on their way to Rome.
In his office as archdeacon, Bernard had the charge of caring for the poor and travelers. For their convenience and protection, Bernard founded a canonry and hostel at the highest point of the pass, 8,000 feet above sea-level, in the year 1050, at the site which has come to bear his name. A few years later he established another hostel on the Little St. Bernard Pass, a mountain saddle in the Graian Alps, 7,076 feet above sea-level. Both were placed in charge of communities of canons regular, after papal approval had been obtained by Bernard during a visit to Rome. The new community was placed under the patronage of Saint Nicholas of Myra, patron saint of travellers. - Bernard of Menthon
Wikipedia sources that the follow centuries had so many churches and monasteries built in them, but the actual number are truly unknown:
The above lists are very incomplete due to the unknown? How many Early Church structures have perished with no historical or archeological evidence to support its’ existence. This answer is obviously unanswerable.
The following articles may be of interest: