well think. Naruto himself has always had huge chakra reserves being Uzumaki and all. Using his chakra, he was able to creates thousands of clones, when to most ninja, 4 is pushing it.
Not to mention the half Kurama in Naruto. Even half of Kurama is seemingly never out of chakra. Although due to the scale of that battle has has run out, but quickly regains it.
Then there is Sage mode which uses the nature's energy, so Naruto can spam it.
Alternating between all three, if used right, it makes sense how Naruto is nonstop fighter.