Tennis racket bags are somewhat confusingly categorised by the maximum number of rackets they can carry. However, that is not to say that is the number of rackets you must carry, as most users will carry 1–3 rackets and use the rest of the space for other gear.
Common tennis bag sizes:
3 Racket Bags: Single compartment, ideal for recreational players who only carry racket and balls.
6 Racket Bags: Two compartments, ideal for club players who carry rackets, tennis shoes, towel and drinks.
9-15 Racket Bags: 3 or more compartments, ideal for tournament players who carry several rackets, change of clothing and other gear.