This was posted in the forum a long time ago and I guess it's still relevant now. Maximum attendances relative to level:
L1 - 800

L2 - 2,500

L3 - 6,000

L4 - 11,500

L5 - 19,000

L6 - 28,000

L7 - 40,000

L8 - 53,500

L9 - 69,000

L10 - 86,500

L11 - 106,000

L12 - 127,500

L13 - 151,000

L14 - 176,500

L15 - 204,000

L16 onwards - 260,000