I manage my fonts through FontExplorer X. Today I noticed that I have 770 fonts activated. All legitimately licensed, by the way. The Adobe CS family installs a bunch, I've purchased and have received many more from friends: I could easily activate a 1,000+ fonts. (I'm referring to individual font files, not 1,000 font families).
At different times I've heard that it's not a good practice to have too many fonts active at once, but I can't find any guidelines as to what constitutes too many, and I really don't understand where the overhead is spent. Is it simply the font-listing UI's in different apps, or does every active font actually bring some overhead at a general OS performance level?
Let me know if my inquiry is unclear.