Many spell not need hp its need mana
If you go over your manager limit, your HP will be drained.
Mana*
Not always the case. Since this is so heavily based on D&D it can also just mean you can't cast spells any more. Or was it stated that your hp would then be drained?
Also, to the OP:
This could be one of those "the MC is fucked no matter what he does" isekai. Where he does survive, but not because he's OP. Instead he could survive because of shady/clever tactics and luck.
His HP probably will not exhaust or does not have limitations.. i mean with all the isekai ive read thats practically the case.. the HP wont go up so it means he can use so many spells or mana and it wont have any effects or toll on his body.. that's just my guess.. taking from what ive read so far from othera (〜￣△￣)〜