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