how many kids live on campus?

I’m going to assume most kids live on campus freshmen year.
What about the rest of the years?
I’m getting fin aid, so it’s going to be cheaper for me to live on campus than to live in my own apartment.

All freshmen live on campus. After that, there are a number of on-campus housing opportunities for upperclassmen. I don’t know the exact number or percentage, but I’d say a lot (probably the majority) of sophomores remain on campus, while fewer juniors and seniors do. But it certainly wouldn’t be abnormal for you to remain on campus all four years. This site gives good descriptions of all the housing opportunities: https://living.sas.cornell.edu/live/wheretolive/housingoptions/index.cfm

For some people it is actually cheaper to live in an apartment than on campus, reason being you could find cheaper housing and cooking for yourself could be less expensive. I am not a guru when it comes to FA, but I believe they will factor in your living expenses whether you live on or off campus when they calculate your FA. Check with them.