Interesting. Last I saw, Khyber and Argo were the two most populated servers by a considerable margin. Going by those numbers, here is some more speculation:
Assume the average account pays $4/month (even I do that, and I haven't purchased any points since February), that makes $340k/month from Khyber alone, or $4 million a year. With 6 servers, we can safely assume that Turbine is bringing in at least $20 million a year from DDO.
At 85k accounts playing an average of roughly 1 hour/day, that means there are roughly 3500 accounts on at all times. Adjusting for peak/off times, there are probably 7k+ accounts on during peak times and maybe 1k during off hours.
I'm not sure what sort of resource requirements an active player has on the server, but given the amount of lag we see during peak hours, I doubt the servers are specced to handle much more than an additional 30% of what we currently see, which would mean Khyber can probably support another 25k active monthly accounts. If we assume Khyber and Argo are the most populated, you can probably safely estimate that Turbine can support another 200k active monthly DDO accounts before we would see another server.
All of this, of course, is purely based off my own speculation