The number of fonts in any design piece should be no more than 3.
One font could include the variations within the font “family”. ie Italics, normal, bold, bold italic of one font, create a font family. The addition of a display or feature font should be restricted to one only.
The fonts used for any particular piece should be derived (first option) from any design elements that must be included. ie any logos that feature in the piece would influence what fonts are used for the document (where ever possible).