This is from Ryujin/Copyninja also known as Logan. He had an interest in this topic but still has yet to get his forum application approved.
So, on behalf of all my experience on nearly a dozen RP sites, both Forum based and LOTGD based, my opinion would be that there isn't a 'hard cap' set, but more of a moral limit. How I saw many of the high tier players run with multiple beasts was that the strain of all of them being in their control caused the character to have psychosis occur. That being said though, it could be similar to what I myself did while 'hoarding' the beasts on a few of those servers. I gathered all of the beasts and the day after the final RP occurred, I called for a Kage Summit, and gave each country several beasts, and enforced that they not subject their enemies to a war using the beasts for X amount of months. If that were to occur on SL, you could have the Kages house the beasts within the village, and safeguard them for two months before it is sealed into a host, allowing 'newbies' the ability to interact with the beast, and maybe set them on a path to someday fight and attain one of them. In doing that, it could create an influx of RPers striving to show how they measure up, which would push the current 'OP' players to a new summit and the option to teach the next generation of players, or turn the place they once looked forward to being, in turn, becoming stagnant and slowly die off due to inactivity.
Granted so far, if it seems like a ramble, I don't mean to do so, the days of little rest due to work or helping friends with several games has caused both my 'filter' and brain to become nearly burned out. In closing, I vote that we don't set a concrete rule, but more of a subtle hint to the population that if you strive to gather all the beasts, do so with people you enjoy being around, and see yourself even wanting to fight beside and protect, like how the Spartans protected their fellow soldiers. But the most important thing to take from SL, is to enjoy the community and enjoy the overall experience and veiled lessons in writing skills and even the movement of the human body, by acting out the physical movements of each post, seeing and feeling the strain it may put on the body to add to the realism of each fight.
P.S./ TLDR; I vote that we simply enforce the idea that we limit the player ratio to 1:1 and allow groups of individual players, not alternates, to gather them, much like Akatsuki did, but also ensure they don't simply overwhelm combatants vying to attain the beasts.