55 different OSes installed on a Powerbook: The only catch is that nearly all are running through Virtual PC.
There are almost all that I need, like, and use. Some are there for nostalgia. Some are frivolous (consider EmacsOS), and maybe even ludicrous (consider “HanoiOS within VMware within Linux within Virtual PC within Mac OS X”). If you must have a number, well, there are screenshots from at least 55 (perhaps more) different systems.
I’m sure you could do something similar on x86 hardware, but the fact that he is running these on a Mac laptop is pretty impressive. The great thing about using VPC for this is that if something gets corrupted and he needs to uninstall everything he only needs to uninstall VPC.