So maybe you can carry 15 seconds worth, and only fire it 5 seconds at a time before some sort of 'reload' animation plays?
Irl you can't 'reload' a fire extinguisher, and doing it ig it's not really realistic.
However, even if you limit the ammo (the same way you can only carry 5 grenades) it's still a bit OP but helpful at times if you get outnumbered in fights.
My opinion still remains, using fire extinguisher in fights puts you in danger because if you miss in any way (or the opponent is lagging), the opponent has a huge advantage as you are pretty much unarmed. He will take all of your armour and maybe half of your health or even kill you if you are not quick after failing to spray with the extinguisher.