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