If someone were to have more then one copy of the Descent base game, would it be possible to place 12 regular beastman on the map, even though the rules say otherwise?

you cannot do that

Possible yes! If you have 12 beastmen you can put them on the board. However the heroes get to play 8 characters. Better stick to one set, hard to get a game of 9 people going.

The original question is kind of like : is it possible to do something that is against the rules... of course it is physically possible to place 12 Beastmen on the board, but it is against the basic rules of descent.... anyhow you can houserule whatever you want...

This surely has to be a case of someone settling a rules dispute from their group by agreeing to raise it in the forums, despite knowing full well that they are in the right.

Jonny WS said:

When the rules say that you're limited to the number of figures available, I think it's pretty clear they meant "available in one copy of the game." All of the rules are written on the assumption you are only using one copy of the game. Sort of like how they're written on the assumption you're playing in Earth standard gravity (it's possible for astronauts to go play in freefall, and they might face some interesting issues getting figures and tokens to stay in place, but the rules aren't going to take this into account explicitly.)

If you want to house rule that the OL can use more figures as long as he has them available, that's your business. It will probably make things harder on the heroes (although it will also make it easier to hit multiple things with AoE attacks.) You could just as easily house rule the use of Warhammer figs as extra beastmen, though. Same degree of legality - house rules.