Lorewise no, In gameplay yes. Ultras are the poster boy for the entire 40k Universe. People WILL want to play them and they WILL join it en masse. You will be seeing a lot more than a thousand ultras trust me.
Yes, they are "poster boys", but there is not only Ultramarines chapter in game. People are different and we WON'T see more than 1000 Ultramarines on ONE battlefield (Of course on another battlefield will be Ultras,too), because there would be other chapters there.
it is unlikely that there will ever be more than 1000 at a single battle anyways... so i wouldnt worry about it...
Oh yeah totally, There won't be more than the full chapter on a single battlefield (I think). Artificial limitations on battle numbers is a bad thing, also making separate copies of environments for different groups of people is also a bad thing. The game is about planet conquest so there should only be that one copy of the planet, not multiple instances on the one server.
i think the point is that even though there is no limit... lag and the amount of poeple in 1 place will limit it self.. i doubt we will see more than 1000vs1000 which is still HUGE and of those 1000 maybe 400 will be the same chapter...
If it's 1000 vs 1000, then I refuse to buy this game. For me anything over 16 vs 16 is not enjoyable at all.