I created some sample data to test this out. My assumption is that you could have the same employee in multiple months, so instead of doing a distinctCount, I did a Count. This way you are counting the employee more than once.
I created a measure to get the the employee count:
Employee Count:=COUNT(Sheet1[Employee Number])
I then created a measure that would give me the employee count by months (this is using the previous meausre):
Employee Count by Month:=CALCULATE([Employee Count], all(Sheet1[Date]))
Create a measure to get the total count of months (this is removing all filters on Sheet1, so be aware. But it works in this scenario):
Total Month Count:=CALCULATE(DISTINCTCOUNT(Sheet1[Month]),all(Sheet1))
Get the overall average:
Overall Average Employee Count:=DIVIDE([Employee Count by Month],[Total Month Count])
Hope this works for you.
