Some computer systems will start to slow down quite a bit as you add more and more fonts. Keep in mind that this can also affect Photoshop's loading time. Generally you want to keep the number of fonts to under 100 for best performance. I usually keep mine in groups and then load a particular group when needed.
Also be sure to examine the "rights" granted for each font. Some are for non-commercial use only. Also some of the fonts are actually fully copyrighted, but have appeared on these type of sites because they were submitted by someone who thought it looked cool (and who "forgot" to mention where it came from). But before using fonts like this in a commercial venue, you'd best make sure that you have the correct rights, or you could end up with a product that might have to be recalled.