Using the metronome

SWS action “Insert click track” will automatically add a click track to a new track. From here, you can adjust volume, slice, and route the click.

From this track, you can change playback speed in order to have it count subdivisions.^[ Metronome and Click Track in REAPER - Beginner REAPER DAW Tutorial]

Another way to use subdivisions is to Insert > Tempo/time signature change marker and create an 8/8 or 16/16 time signature.