Can I change the number of beats for Drum Roll?

Yes, you can change the number of beats for Drum Roll using the MIDI Learn function.
The number of beats you can use are:
1/16, 1/8, 1/4, 1/2, 3/4, 1, 2, 3, 4, 8

 

■How to assign functions using the MIDI Learn feature
rekordbox has a MIDI Learn feature. You can use it to change the buttons or knobs that features are assigned to. For details, see the MIDI Learn Operation Guide [rekordbox.com].