Air Defense at Town Hall 18: 4 of them, max level 16. It unlocks at Town Hall 4. Max level per Town Hall, with how many you can build in brackets: TH4 2, TH5 3, TH6 4 (2), TH7 5 (3), TH8 6 (3), TH9 7 (4), TH10 8 (4), TH11 9 (4), TH12 10 (4), TH13 11 (4), TH14 12 (4), TH15 13 (4), TH16 14 (4), TH17 15 (4), TH18 16 (4).
Tap a Town Hall to open this item there. Dashed: not available at that Town Hall yet.