But since there are so many fpga families and subfamilies, their capabilities, even for the largest devices, greatly differ.
But what is this number good for? Just for research?
A system with many minimal instances of MB-processors seems like a waste of ressources to me.
The PicoBlaze (especially for Spartan6) may be a better choice for that purpose, since the balance between logic and BRAMs is better. But it depends on your application and design intentions, of course.