Wizard Tower at Town Hall 8: 3 of them, max level 6. At Town Hall 18 the cap is 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.