No hard limit.  All related to memory and server performance.

Jeff

DotNetNuke was created for the multi-portal case, but we've never been in a position to do the kind of testing necessary to place upper bound recommendations on this, there are simply too many variables.  Just this week at OpenForce we met users implementing as many as 5,000 portals in a single instance.  Now, I'll tell you that this is NOT a recommended practice (indeed they are having some challenges).  No matter what we do there will ultimately be an upper bound based on lots of factors... but in general, running multiple portals per instance is a business as well as technology decision.  Unless the portals have a specific need to share the same database, splitting out into a manageable number of installations is a wise and safer strategy for planning purposes.

Scott Willhite, Co-Founder DNN