As DWThomas said, if I'm just doing a few prints and have just mixed fresh fixer, a single fix is fine.
Most all of the time however, it is a double-bath fix. I fix prints for half the time and then they go to a holding bath. Near the end of my printing session, I review and choose my keeper prints, which get their second half of fixing in the second finishing fixer bath, hypo clear, rinse, and still water wash.
When the first bath is nearing capacity, I replace it with the second fixer.