Press [Edit / Stamp].
Press [Stamp] twice.
Press [Background Numbering].
Enter the number to start counting from with the number keys, and then press [].
Press [OK] three times.
The numbers appear to overlap the copied image in some cases.
For details, see Stamps.
You can change the settings for Background Numbering under [Background Numbering] in User Tools. For details, see Background Numbering.