Cumulative count

Hi All - just a quick query which is driving me crazy. I've attached a workbook with an abbreviated sample of what I mean (I'm sorry - the data I have is confidential, so I can't post what it actually is). What I want to do is do a cumulative count of the words that I have in column A (in my sample, letters). I just did column C manually, but in my actual document I have 500,000 rows which I want to do this to and what to produce something like column C. I don't know if they're problems, but there are repeats in column A as well as gaps where there are just spaces (they aren't actually spaces because they're formulas which have produced ""). Sorry if this is unclear and thanks for any help!

I mean "want to produce something like column C." Sorry!