Slots are the total number of players a coach can recruit and support through admissions. The number of slots are negotiated with admissions, and will vary from year to year. When a coach is interested in recruiting a player he will get a pre-read from admissions on the player before he invests much time. Receiving coach support through admissions does not guarantee admission, nor guarantee a roster spot.
Edit - One NESCAC had 8 slots for 2018's, which would be simliar to some of the Ivy's