I don't think there is a limit on how many systems we can boot with Grub.
If there is such a limit then I have given up at about 150 systems. However as we can "nest" Grub menus together like a network or a chain and if one menu boots 150 then about 7 menus can fire up 1000 systems.
I am reasonably certain the sky is the limit as far as Grub is concerned.
Mind you all boot loader menus can be nested (for example one can boot to a NTLDR, then jump to a Lilo , then hoop into a Grub, then back to another NTLDR in an endless loop and select any system to boot on its way) so there has never been a limit of how many systems a user can boot.
The word "dual boot" is a funny term to me because I haven't met one boot loader that only boots 2 systems. The minimum is 4 by Dos's MBR because it can boot from any one marked "active" from the 4 primaries partition. WinXp is the least supportive among the modern boot loaders but can still do 10 systems.