Dungeons And Dragons Online: DDO News: Server Caps

Nik Davidson drops by a thread on the official forums to say the target concurrent number of players on servers is 1000. He elaborates on this a bit:

The 1001th person to try to log in won't get a "server full" message, it's just a target, a guideline. We feel like the best gameplay experience will be for average peaks in the 1000 range, so when numbers get near or above that, we'll react. But not by closing a big iron door. And again, we haven't been able to properly playtest this - that's one of our beta goals. So it might change.

It's never fun to get locked out of a server your friends are playing on, so hopefully that will be avoided at all costs. Nik also drops a haiku about it in the same post which you can read here.