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