...the below is a guide, but in any case a font management program is invaluable for overall system stability...
Macintosh System 9.x allows you to open 512 suitcases at once. Macintosh System 8.x and earlier only allow you to open 128 font suitcases at one time. If you exceed these limits, not all the fonts you install will work.
Mac OS X these limits don't apply.
Windows NT, 2000 or XP
These have no limitation on the number of fonts which can be installed, although the boot time will be increased due to the requirement to read the information for all those fonts while the system is starting up.
Windows 95/98/Me
You can install a maximum of approximately 1000 TrueType fonts in Microsoft Windows 95, Windows 98, and Windows Millennium. The exact number of TrueType fonts you can install varies, and depends on the length of the TrueType font names and file names.
Installed TrueType fonts are listed in the registry and in the Graphics Device Interface (GDI).
The Registry
All font files are registered under a single key in the registry, and a registry key cannot exceed 64 KB in Windows 95. If font names average 20 characters in length and font file names average 10 characters in length, the maximum number of TrueType fonts you can install falls between 1000 and 1500.
If a TrueType font file is located in a folder other than the Fonts or the System folder, the full path to the font is included in the registry, using up more space in the key and reducing the number of fonts you can install.
The GDI
The GDI contains an internal list of fonts, with 10 KB reserved for font file names. If font file names average 10 characters in length, the maximum number of TrueType fonts you can install is approximately 1000.
You can use all installed TrueType fonts simultaneously and you can print them in the same document.
Note that the Windows 95 Resource Kit incorrectly states that there is no limit to the number of TrueType fonts you can install in Windows 95 and Windows 98.
Andrew