Organizations need about a dozen D. 7 for the NHL (and 5+ for the A).
Realistically, teams will use 7-10 D in the NHL. So three or so are needed for injury call ups; and hopefully most of those guys are waiver exempt.
(Now, you probably do want 1-2 guys who have the needed experience to share with the defensive A kids, and they'll probably have to clear waivers, if they won't just sign an AHL deal.)