Every level has attendance limit. That limit isn't related with ticket prices and number of fans. You don't have to build new seats and stadium until level 3. Build infrastructure instead (hospitals, gyms, improve terrain etc.)
Level1 - 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 (and higher) - 260,000