dzezy here i find useful a mod to close the topic and say to the vent thread ... is about each level gap for supporters !
let me give you an example :
Lv3 17k supporters on a 50k stadium = 5% if the lvl gap for supporters is 17k !
lv 8(me) 53,5k supporters on a 73 k stadium = 5% possesion because my level gap is max 53,5k !
lv 5 20k supporters on a 30k stadium = 0% possesion because the max supporters you should have on lv 5 can be 30k !

i do not know exact the limit of supporters but you just saw a stadium limited by the game because some of us we build stadium with tokens or on camping level !