That number will vary by school. When you initially accept the job, you will see the number of scholarships available (see image).
As you progress through subsequent seasons at that school, you'll have various numbers of scholarships to fill as kids graduate, transfer, quit, leave early for the NBA, etc. Each school has 12 potential scholarships to fill each season.
Important Note: When recruiting you may not recruit more than 6 players per class, even if you have more openings.