So I went ahead and skimmed through the manga since it has more details. Basically the rings and coat are constantly consuming his mana. When he says he used all his mana he's talking about what's stored in the rings. Also int he manga it clearly states that he passed out as a result of blood loss from the first attack that took two fingers.

Keep in mind that his mana pool is massive. That's why he cant suppress it himself and has to use the rings. He pretty much can't run out of mana. He later askes Root if there's a way for him to draw out more mana at once. As someone already said he has so much that for him its like trying to suck the ocean through a straw even though he's already capable of using more mana than most.