Hidden Tesla at Town Hall 18: 5 of them, max 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.