They fund however many they think are qualified.
Techstars directly copied the model, Seedcamp is inspired by YC and invests a little more for a bigger stake.
Two VCs, Lightspeed and Highland Capital have summer programs for student entrepreneurs.
Don't forget YEurope.
Apparently, Hacker News user "enki" started this one if I'm not mistaken.
Also, who are the other organizations like YCombinator?