Skeleton Trap at Town Hall 18: 4 of them, max level 5. It unlocks at Town Hall 8. Max level per Town Hall, with how many you can build in brackets: TH8 2 (2), TH9 3 (2), TH10 4 (3), TH11 4 (3), TH12 4 (3), TH13 4 (3), TH14 4 (4), TH15 4 (4), TH16 4 (4), TH17 4 (4), TH18 5 (4).
Tap a Town Hall to open this item there. Dashed: not available at that Town Hall yet.