the regular WO join, if any do. Bronco determines that 105 will attend camp. After tryouts on the first day of class, he'll add the PWO and walk ons, bringing the total number on the team to 123. 18 are added to the 105 after camp.
BYU does not find 18 players at the public WO tryouts, they'll usually have 12-15 PWO ready to go and maybe find 3-6 at the public tryouts. Usually the public tryouts are to fill in gaps where scout team guys were hurt in camp.