I want to have a running total that updates each time i use an option from a "pick list"

Ok so this might sound a little weird but basically what I want to happen is have a running total at the top for each month of the year
so lets say I have 10 slots each month I want to create a drop down that lists each month of the year
so say every time I choose "January" I want it to deduct from a running total at the top so I know how many of the 10 i've already used "January" as an option
if tried about every =IF formula I can think of so please someone help