Seeking Air Mine at Town Hall 17: 9 of them, max level 7. At Town Hall 18 the cap is level 8. It unlocks at Town Hall 7. Max level per Town Hall, with how many you can build in brackets: TH7 1, TH8 1 (2), TH9 2 (4), TH10 3 (5), TH11 3 (5), TH12 3 (6), TH13 4 (7), TH14 4 (8), TH15 5 (8), TH16 6 (8), TH17 7 (9), TH18 8 (9).
Tap a Town Hall to open this item there. Dashed: not available at that Town Hall yet.