Dark Elixir Drill at Town Hall 9: 3 of them, max level 6. At Town Hall 18 the cap is level 11. It unlocks at Town Hall 7. Max level per Town Hall, with how many you can build in brackets: TH7 3, TH8 3 (2), TH9 6 (3), TH10 7 (3), TH11 8 (3), TH12 9 (3), TH13 9 (3), TH14 10 (3), TH15 10 (3), TH16 11 (3), TH17 11 (3), TH18 11 (3).
Tap a Town Hall to open this item there. Dashed: not available at that Town Hall yet.