I have 3 that I want to back up to a TC. I don't have a TC yet and want to be sure that it will work...
AFAIK, the only limitation is the space on the Time Capsule. If you have a Mac Pro with 4TB of storage space, you may only be able to backup that one Mac on a TC, whereas 5 Mac minis with 80GB of storage space each wouldn't be a size issue at all.
Another limitation could be wireless bandwidth. If you have a bunch of Macs that coincidentally try to backup lots of data at the same time, you could run into problems there. The odds of backups happening at the same time is probably pretty remote.