Jack -- I'm just guessing, but the limit of fonts is partly going to be determined by the length of the drop down box. For practical purposes, that's around a dozen. I can't imagine needing / wanting more, personally. A couple of text serif, a couple of text sans serif, maybe a funky font or two as well (like Comic Sans) ....
To be honest, the Kindle 3 has two and half fonts: a serif, a sans serif and a condensed variation. With Kobo shipping now with five (or is it seven?), that still leaves space for three or four custom ones. Cyrillic, anyone?