Seeking Air Mine at Town Hall 18: 9 of them, max 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.