Does Dragonball super have only 1 underworld? Or one per universe? One thing I'm not sure about with the introduction of the different universes is: Does Dragonball super have only 1 underworld/afterlife? Or 1 per universe?
I'm pretty sure that each universe has its own heaven and hell. We know that a universe is divided into four quadrants and each has its own Kai. Then there is the Grand Kai who governs them. After that we have the Supreme Kai who looks over the entire afterlife and the universe.

There is an extensive system of Kais to govern the entire universe, including the afterlife. It can be concluded that because each universe has its own Supreme Kai (and of course, the God of Destruction), it also has its own underworld.