Depending on the hub, maybe even one cross for a 20". I've got a Rohloff in a 20" that I built one cross, and it's working out fine. Two cross was going to be too much angle.
A Nexus 8 or Sram 9 doesn't have that much smaller flange than the Rohloff, so if it ever came up, I'm sure I'd do that one cross as well.
On the other hand, I once built a 5 speed Sturmey Archer 36 spoke into a 20", and that one worked OK at two cross.