Elixir Storage at Town Hall 4: 2 of them, max level 8. At Town Hall 18 the cap is level 19. Max level per Town Hall, with how many you can build in brackets: TH3 6 (2), TH4 8 (2), TH5 9 (2), TH6 10 (2), TH7 11 (2), TH8 11 (3), TH9 11 (4), TH10 11 (4), TH11 12 (4), TH12 13 (4), TH13 14 (4), TH14 15 (4), TH15 16 (4), TH16 17 (4), TH17 18 (4), TH18 19 (4).
Tap a Town Hall to open this item there. Dashed: not available at that Town Hall yet.