My bad!
Suppliers do not have spend in the Selected Month ( ie September) but have spend in one or more of the previous 12 months.
I need the formula to group them by the last 12 months spend even if they do not have spend in the selected month.
Months Purchased In 12 months back = // Number of months that each particular client purchased in within the last 12 months from the date selected.
VAR MonthlySales =
CALCULATETABLE(
SUMMARIZE( 'CALENDAR', 'CALENDAR'[Month & Year], "SalesAmt", [Total Net Sales] ),
DATESINPERIOD( 'CALENDAR'[Date], MAX( 'CALENDAR'[Date] ), -1, YEAR ) )
RETURN
COUNTROWS(
FILTER( MonthlySales, [SalesAmt] > 0 ) )