In the attached, in the vicinity of cell S3 of the Summary sheet there's a small pivot table which gives you the total customer references according to the existing Month slicer choices.
In cell S1 there's a formula giving the same data from your pivot table.
If you select any cell in the reference column of your pivot table and then go to the ribbon, Pivot table tools, Options, Active Field section, and click on Expand/Collapes Entire field you'll see just the subtotals and Grand Total.
BTW, your long formula (to get quarters) of:
=IF(MONTH([@[Comp Date]])=1,[@Year]&"-Q3",IF(OR(MONTH([@[Comp Date]])=3,MONTH([@[Comp Date]])=4,MONTH([@[Comp Date]])=5),[@Year]&"-Q4",IF(OR(MONTH([@[Comp Date]])=6,MONTH([@[Comp Date]])=7,MONTH([@[Comp Date]])=8),[@Year]+1&"-Q1",IF(OR(MONTH([@[Comp Date]])=9,MONTH([@[Comp Date]])=10,MONTH([@[Comp Date]])=11),[@Year]+1&"-Q2",IF(MONTH([@[Comp Date]])=12,[@Year]+1&"-Q3","")))))
can be shortened to:
=[@Year] & "-Q" & CEILING(MOD(MONTH([@[Comp Date]])+6,12)+1,3)/3
and the current formula (for the month) of:
=CHOOSE(MONTH([@[Comp Date]]),"Jan","Feb","Mar","Apr","May","Jun","Jul","Aug","Sep","Oct","Nov","Dec")
can be shortened to:
=TEXT([@[Comp Date]],"mmm")