Every level has a fixed number of fans attendance. so even if your stadium capacity is 100k only 2.5k will attend. every season the attendance goes higher but it doesn't matter since the main purpose is to get +5% possession .. and please be aware that ticket price affects the attendance as well .. so keep checking the price till you get the maximum profit out of it .. also the results of the matches matter as well .. let's say you lose few games in a row at home .. there is a chance that the attendance will be lower than before which means you have to get better results and lower the ticket price till you start winning again.

I hope this helps.