If you're talking about the column's initial value, try something along the lines of:
COUNT(SELECT(Table[User], AND([Workout Day] = [_THISROW].[Workout Day],[Exercise Name] = [_THISROW].[Exercise Name])) + 1
If this is a child table, you might be able to utilize a technique like:
COUNT([Parent ID].[Related Child Rows]) + 1
Additional information in Serial Numbers, If You Must - Google Cloud Community .
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