It depends on how much your willing to spend on them. Having more vets allows you to remove rookies and experts eventually and save weight while still staying at/above the ACC/RLD caps. A pair of IJN 10/10 BVE(Boosted/100vets/Full experts) gunners will have capped both ACC/RLD by the time they are using the BB4 guns. Im not going to do the calculation's right now, but I think even 9/9BVE IJN gunners would reach both caps by BB4 :)
Having better bases, and/or more than 100 vets simply allows you to reach the caps a little quicker. Also allowing you to shave crew off and lighten up your gunners while staying above the caps
