From the home screen, navigate to:
Copy > Advanced Options > Separator Sheets
Note: Set Collate to “1,2,3 ‑ 1,2,3” to place separator sheets between copies. If Collate is set to “1,1,1 ‑ 2,2,2,” then the separator sheets are added to the end of the copy job. For more information, see Collating copies.