Let me add a little bit to this discussion.
I have clients with 200 portals, that work well, and I have other customers with 200 portals that can hardly function. I'll to to the extreme, and I have a customer with 5,700 portals, and they manage to get sub second response times for viewing, but administration sucks.
What is my point?
Well, yes, DNN can most likiely handle it, but with that many portals a few things are key.
1.) DNN's admin systems are NOT necessarily user friendly at that level
2.) Many third party module interfaces are not optimized in a manner that works well with large portals
3.) You MUST, MUST, MUST, MUST be on a dedicated, stable environment, with a good amount of ram, to even think about pulling this off. Ideally, if at the 10K portal range, a minimum of a dedicated db server and a dedicated webserver.
4.) Be prepared to have to start digging if you have performance issues.