Air Bomb at Town Hall 18: 8 of them, max 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.