Well, 'The Babysitters Club: Dawn and the Impossible Three' is also worth reading. It shows how the club deals with difficult babysitting situations, and you can see the different personalities of the club members shine through as they work together.
Friendship is a major theme. The girls in the club support each other through various situations like dealing with difficult clients or family issues. Another theme is responsibility as they take on the task of babysitting and all that comes with it.
It's hard to give an exact number. The number of graphic novels of The Babysitters Club can change over time as new ones are released. Your best bet is to check with a reliable source like the publisher or a comprehensive book database.