Elixir Collector at Town Hall 4: 4 of them, max level 8. At Town Hall 18 the cap is level 17. Max level per Town Hall, with how many you can build in brackets: TH3 6 (3), TH4 8 (4), TH5 10 (5), TH6 10 (6), TH7 11 (6), TH8 12 (6), TH9 12 (7), TH10 13 (7), TH11 14 (7), TH12 15 (7), TH13 15 (7), TH14 16 (7), TH15 16 (7), TH16 17 (7), TH17 17 (7), TH18 17 (7).
Tap a Town Hall to open this item there. Dashed: not available at that Town Hall yet.