Actually, I'm pretty sure you can install more than 27 OS's as long as your boot loader supports it. I seem to remember a commercial one that let you go upto a hundred OS's. However, you won't be able to install more than 4 BSD OS's per hard disk because they can't install on extended partitions. Sirdirge: they'll know which kernel to load because the entry in the boot loader will tell them.
Cheers,
cgkanchi