Excel does have limits on how many different formats can exist in a single
workbook. For example, in Excel 2003 you are limited to 56 different colors,
4000 cell styles, and 200-250 number formats. You can see a full list of
Excel's limitations by searching for "specifications" in Help.
Almost all of these limitations are vastly increased in Excel 2007. Such
as, 16 million colors, and 64000 cell styles, however number formats are
still limited to 200-250.
So, in answer to your question, your best option would be to reduce the
number of different formats you are using. But, upgrading to 2007 may help
as well.
HTH,
Elkar