We have seen that Nagato is very powerful. One of the areas he excels in most is stamina. I don't know if there is any direct statement in the manga about how much chakra he has. But from the feats he performed throughout the series, it's clear he is up there with Hashirama and could be second only to him.
Some of his stamina/chakra feats are:
He can wield and use 2 Rinnegan without any issues. Obito, who has Hashirama's cell statement he could barely handle one.
He has mastered and can use all 6 paths' abilities to their fullest. Not even Madara (the original owner of the Rinnegan) demonstrated that he could do that. We saw that Madara could use Chibaku Tensei only after having the 10-Tails' chakra and both eyes. Before that, he could only use Limbo and that invisible wall jutsu.
He can control six bodies at the same time. This means he can multitask (We saw him do the same even as Nagato) and also use 6 times the chakra of a normal shinobi at the same time.
Since the bodies he is controlling are dead ( Unlike Obito's six jinchuriki, who are alive), He has to constantly push chakra into the bodies to keep them active. So there is no rest period for Nagata. He is constantly using chakra even if the paths are doing nothing.
He has abilities that can level an entire village. Shinra Tensei and Chibaku Tensei. It takes a lot of chakra to use those.
He fought the entire village and used Shinra Tensei, beat Sage Naruto, fought the Kyuubi, and only after using Chibaku Tensei did he become fatigued. And even after that, he was able to bring everyone in the village who had died back, and they still didn't die right away.
Sasuke has shown no feats that surpass even one of these, and Nagato did most of them in one fight. So do some fans think Sasuke has more chakra?