View Rules and Best Practices in Page Layout Illustrations: Number of Fonts. The examples demonstrate inconsistency, use of too many fonts, and the use of fonts that lack sufficient contrast.
Two much variety creates a mixed message visually and hides the real message in the text. Fonts should not get in the way of communication. Frivolous use of fonts can make a very important message appear to be just so much fluff and nonsense.
The Bottomline: No hard and fast rule says you can’t use five, six, or even twenty different fonts in one document. However, consistency and readability are important to good design and too many font changes can distract and confuse the reader. Make your font choices carefully and consider how many typefaces will be seen together — longer, multi-page publications, such as magazines, can often tolerate a greater variety of typefaces. For brochures, ads, and other short documents, limit typefaces to one, two, or three.
