I would like to offer:
One credit for: The Babysitters Club: The Truth About Stacey (#3), Kristy's Big Day (#6), and Mary Anne Saves the Day (#4) plus, also by the same author (but not in the series), Ten Kids No Pets.
One credit for: Christopher Pike: The Immortal, Witch, and Master of Murder
4 books for one credit, All are R.L Stine. Fear Street: The Best Friend, The Sleepwalker, The Stepsister, The Face. Fear Street: The Catalina Chronicles: The Evil Moon. Fear Street: Super Chiller: Party Summer, Broken Hearts, The New Evil, The Dead Lifeguard. 99 Fear Street: The First Horror. Fear Street Sagas: A New Fear, House of Whispers.
All of these books are posted on my bookshelf so you can read the synopsis.