Your GM probably does not wish to have more than four players. Have you asked him if you can play two characters?
I'd say the "party limit" is whatever the DM feels comfortable with. However, there are no rules saying that a party must be a certain size.
Interestingly modern parties are smaller than they were in the days of Gygax and Arneson. Back then adventuring parties could be roughly from six to twelve characters (not all PCs however, this would include hirelings and henchmen, which are in themselves quite rare in modern games).
