According to the passage, how many illustrators have worked on the Amelia Bedelia series?
Amelia Bedelia is the main character or protagonist of a series of American children's books written by Peggy Parish until her death in 1988, and by her nephew, Herman Parish, beginning in 1995. They have been illustrated by Wallace Tripp, Fritz Siebel, and the two current illustrators, Lynn Sweat and Lynne Avril. The stories involve Amelia Bedelia repeatedly misunderstanding various commands of her employer, Mr. and Mrs. Rogers, by always taking orders literally, causing her to perform incorrect actions with a comical effect. However, she almost always manages to win everyone over at the end by baking a delicious pie or cake. Amelia Bedelia has been a beloved children's book series for many generations.