I think I would be ecstatic with an 18" spr, a 16" recce style rifle, a 14.5 with aimpoint and 3x magnifier, and an 11.5" sbr with an aimpoint. Then get a suppressor or two and start working on a mountain of ammo.
The real answer here is optimal # of ARs = x+1, where x is the # of ARs you currently own.

