Some schools carry 35 -- others near that number. Most of the rosters are posted on the school websites. The obvious adventage of walk-on players is....they're cheap!
The challenge for the walk-on is getting the coaches' attention.
My middle son walked on at TTU and had a few good bullpen sessions. He noticed that the coaches didn't play close attention when he threw -- they were focusing on their large pitching recruiting class, which is understandable. They had made $commitments$ to those kids. I personally think 17 pitchers is too many....3 weekend starters, 2-3 week-day starters, and 10-11 relievers -- not enough innnings, even with 54 games.
Most schools will reserve a spot or two for walk-ons...just in case. Playing time??? -- another challenge.
My advice for walk-ons at D1 schools ----- focus on finding a place that you will love, even without baseball. Making the team is a huge bonus!