Either one works for me.
How many students are in your college?
How many students are at your college?
Which question should I use when talking to my son?
Wow, Batman, you forgot really quickly about relevant thread titles, didn't you?! A good title for this thread would have been "How many students are in your college?"
Remember - if you don't use correct capitalisation, punctuation and spacing, anything you write will be incorrect.
Unless you make some effort to follow the forum guidelines about thread titles, which have been pointed out to you several times, Batman, you will find that your threads will be deleted unread.
We like to help when we can, but we do expect a little cooperation from members.