The cap is equal to the stadium level that is one level below you, but with the large seat upgrade. Which calculates out to:

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

So, upgrade some of your other facilities as you upgrade your stadium. No advantage to having so many seats and not being able to sell any tickets for them.