If you have several interesting characters you want to develop, then use them. Don't stick in random people just to make the numbers up.
If your sitcom is based around a family, you have the conventional 2.4.
If it is a group of friends then it can be 2,4,6 before it gets too crowded.
If it is an office, we have a central character and a boss, secretary and err... other central character/love interest.
4 is a good number.