Thanks for the new chapter!
Pretty sure he already knows how to increase his mana pool - he probably meant how to properly make use of more of that mana when he needs to.
Anyway, the 2nd season of the anime is now up. It will have 24/25 episodes and I pretty much can guess that a big chunk of the cour will be covering what season 1 omitted (i.e. the parts about the other two isekai'ed characters and everything else related to the hyuman-demon battle). I wonder if the anime will overtake this manga adaptation at some point and cover more of source material before the end of the cour?