Is there an average number of outlets that are wired off of one circuit breaker?
For residential, the National Electric Code (NEC) has no official min.  In Commercial and Industrial you can put 10 on a 15 amp breaker and 13 on a 20 amp breaker.  Local codes sometimes specify a max, so you might want to check if you are going high.

But yeah, thats really low!  Unless there were some dedicated appliances running there, you can definitely add more.  10-12 outlets per 20A breaker is what I normally do.  Now if I know I will be running heavier stuff like TV's, stereo equipt, or small bar refrigerators, I will use less.  Keep in mind some appliances need a dedicated line like full refrigerators, dishwashers, spa tubs, etc.