How do practicing Catholics keep track of all the sins they commit before going to Confession? Do they have to confess all of them or as many as they can remember? Is contrition the sign of having committed a sin, so every time the person feels guilty or remorseful of something (i.e. stuffing the whole chocolate cake slice into one's mouth and leaving none for brothers and sisters), would that be counted as something to confess to the priest the next time that person goes to Confession?
How do practicing Catholics keep track of all the sins they commit before going to Confession?
They don't keep track of sins. Before going to confession they do something called examination of conscience. This is in layman term trying to recall all the sin one has committed.
An examination of conscience is a “prayerful self-reflection on our words and deeds in the light of the Gospel to determine how we may have sinned against God” - Glossary, CCC
There are many ways to examine one's conscience. Some of them are listed in USCCB website.
Do they have to confess all of them or as many as they can remember?
They have to confess as many as they can remember. They have to say all mortal sins that they are aware of, and the number of times they’ve committed them, if they know that number. Venial sins do not have to be named, but can also be said.
Is contrition the sign of having committed a sin?
No, contrition is not a sign of having committed a sin. Contrition is a sign of good conscience but not a necessary one to know that you have committed a sin. Feeling bad or guilty is not a necessity for confession. But will to be sorry is. that is, even if you don't "feel" sorry, you should want to be sorry with your will.
There is an enormous difference between regret and repentance. Regret is simply rooted in emotion. I “feel” bad. This can be a good first step, but it cannot be the only step. And it is not even a necessary step. While there is something very healthy about “feeling bad” for “doing bad” (it shows that your soul is a bit more sensitive to the reality of sin), it alone is not a return to God. That’s what repentance is. In fact, this is what “repentance” means: it simply means “turning”. Repentance is the choice to return to God. You do not need to regret what you’ve done (believe me, that will come with time), but you do need to make the decision that you will no longer continue on in this particular sin.src
stuffing the whole chocolate cake slice into one's mouth and leaving none for brothers and sisters, would that be counted as something to confess to the priest the next time that person goes to Confession?
One can't be a judge to the actions of others. Only God will know. Assuming generic conditions, eating food that is excessive in quantity and eating too eagerly or too soon looks like gluttony (mortal sin), and not sharing with siblings looks like greed (mortal sin). So these have to be confessed.