The amount of nature energy that he is able to store at any give time has to be in an equilibrium with his physical and mental energies, each being at around the same point. With that said, Naruto and Kurama's chakra have, for the most part, combined, not including the chakra from the other 8 tailed beasts. This means that the amount of nature energy he should be able to hold is huge, though his mental energy may not be able to sustain it; therefore he can hold a lot, but not control it all at the same time. With that said, it's good for a long battle.