Right now, in the main official campaign, you are limited to 2 people. With modding, there will be the capability for 4. 6 is right out.
Don't expect the cap to rise, because then you'll spend all your time disappointed it does not, if you're only looking for an MMO you'd be better off waiting at least a year, because there is always the chance things will change after a year.
But don't expect the NWN ability for persistent worlds, unlimited player caps and such for a while because the game is designed more like Baldur's Gate with a set number of people in the story than It is for unlimited hero sizes.
