Counting multiple data blocks in a large spreadsheet

have a spreadsheet which contains 14,800 rows and 6 columns. I have a header row and I am trying to count the number of different S/N's (which are many). I have been using the Subtotals on the Data menu and using S/N as the "as each change in", count as the "function" and Count column as the "add subtotal to". This seems to work for about first 1/8th of the spreadsheet but the rest of the spreadsheet will not count. The process also gives a subtotal of each S/N which I don't need but this seems to work throughout the spreadsheet. I hope this explains my problem sufficiently as I was told today that this spreadsheet is needed ASAP.