For any formal business memo or letter, sticking with one font throughout the entirety of the document is best. You can bold or italicize certain headings to draw your reader's attention to pieces of information, but changing fonts is considered sloppy. The Purdue Owl suggests using Times New Roman or Arial size 12 font.
No limit exists as to the amount of fonts that you can use in informal documents such as letters to friends or homemade pamphlets. In general, you don't want your font changes to interfere with the reader's ability to absorb the document's information. One of the best ways to avoid any difficulties is to stick to a small handful of fonts and use them to differentiate certain types of information.
If you are not sure how formal your document is, or you are having trouble using multiple fonts and making the document look good, stick with one font. A page with a single font may look slightly boring, but it is easy to read and looks professional. If you must change fonts in a professional document, use no more than two different fonts.