If they have 3 crashing the boards, then they'll have 2 getting back on every play. Or it could be 4 and 1, or 5 and 0 (everyone crashes the boards). They'll also say who they want getting back - although there are rules about who gets back if one of the designated guys goes for a layup or something. So, say Haws is designated to be a "get back" guy, but Haws is going for a back door layup, or come off a screen to take a baseline jumper, then there will be a rule as to who fills his spot as the get back guy... maybe the guy who set a screen at the top of the key, or whatever.