Running total in Pivot table (Monthwise incrementation by distinct count)

Ubaid ur Rahman , that's not the same as in your table. If you have distinct count for the one month and another one, total distinct count won't be a sum since some suppliers are repeated in full period. Thus total for two months shall be less than sum of distinct count for each of two months. With that logic that is like

DistinctCountSuppliersTotal:=CALCULATE (
    DISTINCTCOUNT([Supplier No.]),
    FILTER ( ALL ( Table1 ), Table1[Purchase date] <= MAX (Table1[Purchase date] ) )
)

If distinct count only for the year

DistinctCountSuppliers:=CALCULATE (
    DISTINCTCOUNT([Supplier No.]),
    FILTER ( ALLEXCEPT ( Table1,Table1[Year] ), Table1[Month] <= MAX (Table1[Month] ) )
)

I added both, please check in the file.

Or you assume another logic?