How would it be kept to sensible numbers? Maybe limited by 'no more than X on the field at a time' (like field guns now), till the source object (barracks) are destroyed.
Making it a kill would of course limit it, but aside from that Moot, I'm not sure it needs to be limited. After all, there are no limits to the number of Wirbies that can be spawned from a working VH. Also we are talking about rifle caliber MGs, so even if you were to spawn 8 on the field, we still only have the cumulative destructive power of a Spit 1, and since the guns aren't calibrated and firing at the same target, its actually much less potent than that. We are really just talking about oil/coolant hits, control surface damage, pilot wounds, and a few more holes in the fuselage - which is what happened in reality. It was rare that pilots strafing defended military targets didn't come home with at least a hole or two in the airframe.
This is mostly just to add historicity to field defense, give the barracks actual value in field defense, and make it just slightly more difficult to totally silence the ground defense at an airfield.
- SEAGOON