Air Bomb at Town Hall 16: 7 of them, max level 11. At Town Hall 18 the cap is level 13. It unlocks at Town Hall 5. Max level per Town Hall, with how many you can build in brackets: TH5 2 (2), TH6 2 (2), TH7 3 (2), TH8 3 (4), TH9 4 (4), TH10 4 (5), TH11 5 (5), TH12 6 (6), TH13 8 (6), TH14 9 (7), TH15 10 (7), TH16 11 (7), TH17 12 (7), TH18 13 (8).
Tap a Town Hall to open this item there. Dashed: not available at that Town Hall yet.