Nikon D850 Menu Manual page 228

Choose a channel for the master flash.
If the remote flash units include an
SB-500, you must choose channel 3,
but otherwise you can choose any
channel between 1 and 4.
4
f: Set the remote flash units to the same channel as the
master flash.
Set the remote flash units to the channel selected in Step 3.
5
f: Group the remote flash units.
Choose a group (A, B, or C) for each remote flash unit.
Although there is no limit on the number of remote flash
units that may be used, the practical maximum is three per
group. With more than this number, the light emitted by the
remote flash units will interfere with performance.
6
C/f: Compose the shot.
Compose the shot and arrange the flash units. See the
documentation provided with the flash units for more
information. After arranging the units, press the test buttons
on the flash units to test-fire the flash and confirm that the
units are functioning normally. Flash units can also be test
fired by pressing the i button in the flash info display (0 243)
and selecting M Test flash.
7
C/f: Frame the photograph, focus, and shoot.
228 Optional Flash Units