Hidden Tesla at Town Hall 16: 5 of them, max level 15. At Town Hall 18 the cap is level 17. It unlocks at Town Hall 7. Max level per Town Hall, with how many you can build in brackets: TH7 3 (2), TH8 6 (3), TH9 7 (4), TH10 8 (4), TH11 9 (4), TH12 10 (5), TH13 12 (5), TH14 13 (5), TH15 14 (5), TH16 15 (5), TH17 16 (5), TH18 17 (5).
Tap a Town Hall to open this item there. Dashed: not available at that Town Hall yet.