I was at casting last week, checked in 2/7 and a cast member was asking us trivia questions while we were waiting. She said there were about 3,000 CPs for this Spring season.
No cast member has said anything about how ppl are rejected & accepted. However, I am assuming that the number they take is based on need in the parks not based on how much room they have in housing. Also, the head of the programs office did say that he will terminate at least 3 people on the first night alone (for breaking rules), every check in date of each program season. So theres also extra room since people do dumb things and get kicked out.