Wall at Town Hall 11: 300 of them, max level 12. At Town Hall 18 the cap is level 19. Max level per Town Hall, with how many you can build in brackets: TH3 3 (50), TH4 4 (75), TH5 5 (100), TH6 6 (125), TH7 7 (175), TH8 8 (225), TH9 10 (250), TH10 11 (275), TH11 12 (300), TH12 13 (300), TH13 14 (300), TH14 15 (325), TH15 16 (325), TH16 17 (325), TH17 18 (325), TH18 19 (325).
Tap a Town Hall to open this item there. Dashed: not available at that Town Hall yet.