I think it should be based on total number of vehicles for squadrons. Basically, if you have 5 predators in your list and you field a squadron. If you lose two preds you can still field a squadron the next game by adding the 2 remaining preds. If you lose all three preds in battle you can no longer field the squadron with just the two remaining preds. I think doing it any other way really gives an unnecessary advantage to people running squadrons.