Gunboats: 2 Bombers or 3 Fighters
Cruiser: 2 Bombers or 2 Gunboats
Battleship: 3 Bombers or 3 Gunboats
Both parties are consisting of players of average skill (i.e. bombers missing, battleship pilots aren't very accurate, etc), even at that these are very arbitrary numbers. Typically piloting skills vary much from player to player, specifics of ships, loadouts and equipment such as capital ship armor kits can play decisive role in outcome of battle, least not forget teamwork can turn the tides against nearly impossible odds.