Giant Bomb at Town Hall 13: 6 of them, max level 7. At Town Hall 18 the cap is 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.