There is practically no chance that each "DDO game server" i.e. Ghallanda, Thelanis etc., is "one server", as in one physical machine, like you are sitting and writing on "one PC" right now.
Instead, each server is likely a cluster, meaning many servers ("blades") working together, with high-speed fibre optics connecting the cluster members. DDO Instances then get sent to different members - physical machines - by a load balancing scheme. Apart from the impossibility of one machine housing the data power to keep hundreds of quests and raids computing and in memory, this approach also means they can easily scale, add new cluster members or take them away, along with other advantages (if one instance cluster server hangs, the server/s in charge of load balancing might just send new instances to the other members, etc). All housed at some Boston data centre.
What really constitutes a "DDO game server" (apart from the server cluster) is likely a group of databases, containing stuff such as character info, bank data, questing data, guild data for each character and item on each server. So maybe we have one "ghallanda_characters" database, one "ghallanda_loot" database, one "ghallanda_raid_statistics" etc. Or they are all part of one big database per server. There also might be separate chat servers, voice chat servers, Auction House servers etc. for each DDO game server.
Caveat - I don't work with MMO architecture, so I'm just guessing
