Air Sweeper at Town Hall 11: 2 of them, max level 7. It unlocks at Town Hall 6. Max level per Town Hall, with how many you can build in brackets: TH6 2, TH7 3, TH8 4, TH9 5 (2), TH10 6 (2), TH11 7 (2), TH12 7 (2), TH13 7 (2), TH14 7 (2), TH15 7 (2), TH16 7 (2), TH17 7 (2), TH18 7 (2).
Tap a Town Hall to open this item there. Dashed: not available at that Town Hall yet.