Answer:
That depends on the version you have. Up to and including Excel 2003, the standard amount of columns has been 256. They are labelled by letters. After Z you get AA, AB, AC etc. until you get to AZ. Then it is BA, BB, BC and so on. The 256th column is IV. In Excel 2007 the number of columns is 16,384 which means the last column is column XFD.
Excel columns are lettered from A to Z, then continue through AA, AB, AC, AD, etc. Versions of excel prior to Excel 2007 stop at column IV, which if counted results in 255 columns. Excel 2007 columns continue to column XFD which if counted results in 16,385 columns.