Right, I meant no more than three distinct fonts, like Caslon, Garamond and Tahoma (those being fonts I use a lot and were thus the first to come to mind; I don't necessarily recommend using them together). You can usually get away with using variants like Bold, Oblique or Condensed of each of those if you need greater typographical hierarchy than you can get from just the base fonts themselves. Now that I mention it, that concept of hierarchy is a good way to think about things. If a label is of greater importance than others, then it should have some quality that makes it stand out: Size, weight or color. But don't vary those traits arbitrarily, since that will weaken the unity of the picture.
even though contemporary atlases, for instance, use heavy, underline, italics etc do denote different features? though i suppose you can get quite a bit of variety with different styles from the same font family.