A revolver is an advanced firearm that holds up to 6 bullets. Because it's an advanced firearm, you can load it to capacity with a single move action.
Reloading hands automatically loads one weapon with conjurated ammo each round. I would assume this means "load to capactiy," but in parentheses it says "a normal bullet or pellets and black powder."
Does it load 1 bullet or does it load the weapon to capacity?
*emphasis mine
I would read that since it says a normal bullet it would load one bullet.
Using more text than the OP posted, I'd say that it reloads the entire weapon:
Once per round, phantom hands load a single ranged weapon or firearm with conjured ammunition. This ammunition counts as magical for overcoming damage reduction and attacking incorporeal creatures, but is the standard for its type (a normal bullet or pellets and black powder in the case of firearms).
The full text says that it is reloading the weapon (not just a bullet).
The text in parenthesis seems to just be pointing out that the bullets that are loaded are standard bullets, not anything special, like alchemical.