That really depends on the bootloader, lilo, grub or whichever you use. I think I read about a limit of thirty or so, not sure of exact number.
Be careful of what you share between distros though. /boot is OK and I would recommend it, the rest are a bit touchy.
I have had two Mandrakes and two Gentoos at one time. I don't use windoze so not sure how that would work.
Hope that helps a bit.