Use # to represent a digit and the start number separated by "|'.
For example,
If you want to mark your sequence number in format of 001, 002...999, use ###.
If you want to mark your sequence number in format of 01, 02...99, use ##.
If you want to mark your sequence number in format of 1, 2...10..100, 1000... use #.
If you want to mark your sequence number in format of 010, 011..., use ###|10.
See also:
Insert EXIF or other data into text watermark