The change to 12 came in the second set, so that number may have just been upping the previous high. Character numbers were all even numbers back then, so 12 was the next logical increment. I'm sure they felt that
The False Prophet demanded more power than any human. This, of course, was countered by
Michael in the Warriors set (also not a human). If there was going to be a human with that kind of power, then
Samson made sense, hence the promo (I'm not sure when that was released, though).
There could be a correlation to the 12 tribes of Israel, but I think it was just that 12 came after 10 in even-numbered increments.
As far as going higher than 12, that would not have been a need, since they would not get initiative and likely be obliterated.
Taunt has made the numbers relevant again, so I guess it is possible. I'm not sure why an animal would be more powerful than an angel or demon, but they could certainly be stronger than Samson.