=SUMPRODUCT((INPUT!$B$2:$B$20000 = A2)*(INPUT!$A$2:$A$20000 =
MONTH(NOW()) -1))

I would like to count the number of records from the previous month
regardless of what month it is.

So if I run this in October, it will check column A, and give me a count of
all the records for September.

I don't think I am doing this correct.