I don't actually know how many Yeerks would fit into one brain, but I would think that control of the host would just depend on who got there first. IE, if one Yeerk had already settled in when the next one tried to infest the host, the second one probably wouldn't be able to gain control, but if two Yeerks entered a host at once, they'd probably control different functions of the body depending on who landed on which sections of the brain.
Heh, I just got an amusing visual of two Yeerks trying to fit in one head and hanging out of a guy's ears.
Wow, lots of intriguing questions on this thread. In my personal opinion, I think that if a Yeerk tried to infest V3 in human morph, that Yeerk would gain control over him (since Esplin would be incorporated into the human morph's mind the same as Alloran), but when he demorphed that Yeerk would go into Z-space and Esplin would get control again until he morphed human again. Either that, or the Yeerk might simply be destroyed, I'm not entirely sure. Does that make any sense to anyone?