64 teams are in, 271 are out. 32 teams are in based on winners their conference however that conference determines how they win; some by tournament, others by record and tie breakers can vary as well.
Lots of discussion this year that Texas is the Big 12 champion while Baylor is the #1 team in the country. It happened because Texas beat Baylor in 3 games and Baylor beat Texas in 5 games. The Big 12 tie breaker is based on who won in few games rather than some decide based on total points scored. #1 in the country is #2 in the conference.