There is a specific rule about the maximum amount of supporters a Manager can have in a home match. Indeed, this amount is limited by the Manager's level and can't be exceeded regardless of the stadium's capacity.

Here are the maximum capacities that a stadium can reach on these Manager levels:

Lvl 1 - 800
Lvl 2 - 2,500
Lvl 3 - 6,000
Lvl 4 - 11,500
Lvl 5 - 19,000
Lvl 6 - 28,000
Lvl 7 - 40,000
Lvl 8 - 53,500
Lvl 9 - 69,000
Lvl 10 - 86,500
Lvl 11 - 106,000
Lvl 12 - 127,500
Lvl 13 - 151,000
Lvl 14 - 176,500
Lvl 15 - 204,000
Lvl 16 - 260,000