Excel also has no problem with more than a dozen typefaces, with one size/style of each (I just tested it.)
The error message is probably using “fonts” in the technically correct sense for combinations of typeface, size, and style, while the user experiencing the error is probably confused and reporting that they are only using 8 “fonts” using the term to mean typefaces (which is how Excel itself uses the term in the UI, with the typeface selector using the label “Fonts”.) But if you are using 8 typefaces, using multiple sizes and styles of each, you can easily be using over 100 actual fonts.
The error message is probably using “fonts” in the technically correct sense for combinations of typeface, size, and style, while the user experiencing the error is probably confused and reporting that they are only using 8 “fonts” using the term to mean typefaces (which is how Excel itself uses the term in the UI, with the typeface selector using the label “Fonts”.) But if you are using 8 typefaces, using multiple sizes and styles of each, you can easily be using over 100 actual fonts.