Was just talking to my D* rep out about this the other day. As Stargazer stated, 6 is the number ... without any questions asked. You can go more then that, but be prepared for a third degree Q&A and the majority should be connected to phone lines since they will more then likely have them call home to make sure you are not "sharing" your account.
That being said, do not take it for gospel. That's just what one D* employee said on one day. Dependant upon the knowledge/length of employment/common sense of the CSR you get on the phone can make a difference as well. D* and for that matter E* are both well known for having policies that are open to interpretation by whomever you talk to at any given time.