Actually, there's no difference between the Flak tank and AC HWS as far as inflicting damage is concerned.
The Hydra gets 4 TL shots at BS 3, averaging 3 hits per tank. The HWS gets 6 shots at 3 BS, averaging 3 hits per squad. So, given that you want to stay mech and deny enemy their small arms fire, go with the tanks.
And as far as the number is concerned, I'd take 3. As I've mentioned, you're going to get 3 hits per tank. Against AV 11, you have a 1/3 chance to pen, so you'll average 1 pen per tank. Considering each pen only has a 1/3 chance to destroy the vehicle, I'd take 3 tanks, giving you 3 pens, and (hopefully) ensuring 1 destroyed result per turn.
Against AV 12, this is halved, so you'll only get 1 destroyed result every 2 turns.