I am in charge of giving away a number of T-shirts which come in different sizes and colors, but they are limited in quantity. I wanted to create a Google Form for giving the T-shirts away, but I do not want people to fill out the form thinking that they would get a T-shirt color/size combination that other people have claimed already. Is it possible to show the number of T-shirts left in Google Forms, if the user chooses a color/size combination, and decrement this number if the user has chosen it?
Yes, it's possible. You should use an add-on or Google Apps Script to update the remaining quantity.
Maybe the best option is to do the update on on each submission but this will depend on how often you get a submission. Also, the remaining quantity will not be precise all the time.