Do you get the error in Design view when you're trying to add the 6th subreport, or do you get that error at runtime when trying to view/run the report?
I have Access 2003, and I was able to add 7 subreports to my report and it ran successfully. That would lead me to believe that perhaps there's a problem with one of your subreports. Have you tried running that 6th subreport on it's own to see if you still get that error?