Wizard Tower at Town Hall 18: 6 of them, max level 17. It unlocks at Town Hall 5. Max level per Town Hall, with how many you can build in brackets: TH5 2, TH6 3 (2), TH7 4 (2), TH8 6 (3), TH9 7 (4), TH10 9 (4), TH11 10 (5), TH12 11 (5), TH13 13 (5), TH14 14 (5), TH15 15 (5), TH16 16 (5), TH17 17 (5), TH18 17 (6).
Tap a Town Hall to open this item there. Dashed: not available at that Town Hall yet.