Hi there
If you boot with GUID then the theoretical limitation is the GIUD number -- I think that the possible number of GUIDS is larger than the probable number of stars in the entire universe.

A Phsyics guy or mathematician should be able to confirm that.

In practice it would take longer than the expected lifetime of the universe to Load all the possibilities into the menu -- assuming your machine was big enough (or you had access to a "Quantum Computer" in which case you could boot an infinite number of OS's SIMULTANEOUSLY !!!).

HowStuffWorks "How Quantum Computers Work"