For that matter, a SEQuence Field can count up. You could enclose it in a calculation field to step it.

Start numbering at 1, step 7:
{ = 1 + {SEQ MySeq } * 7 - 7 }
This will give you: 1, 8, 15, 22...

Oops, misunderstood "step." Could still use a SEQ field. Put "Step { Seq MySeq }" in an AutoText entry.

But, why not simply use Automatic numbering linked to styles?
How to create numbered headings or outline numbering in Ribbon Versions of Word by Shauna Kelly