Air Defense at Town Hall 13: 4 of them, max level 11. At Town Hall 18 the cap is 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.