I have a runtime for each character. M3, V3, M1, V1, the MilKids, the Young Teens, the Preschoolers, David, SP3, etc. And there's a separate runtime for "Hair," since you can use that with any character. I also put other things that I can use for any character there, such as eye textures.

I also have some "theme" runtimes. Fantasy, SF, Contemporary, Nature, Toon. And there's a runtime for current work, so I can keep stuff I'm using on active projects there, then delete them when I'm done.

It really depends on the way you work. Remember, it's easy to add and remove runtimes. You probably won't want two dozen runtimes loaded at once. If you organize your runtimes properly, you'll be able to load just a few at a time, depending on what project you're working on.