It's tricky to give a definitive number because there are many different interpretations of "based on." For example:
* Adaptations: There are numerous movie, TV, and stage adaptations of his works, such as "The Lion, the Witch and the Wardrobe," "The Chronicles of Narnia," and "The Screwtape Letters."
* Sequels and Prequels: Some authors have written books that continue or precede Lewis's stories, although these are not considered canonical.
* Fanfiction: There's a large community of writers who create stories inspired by Lewis's work, ranging from short stories to novels.
To get a more accurate answer, you'd need to clarify what kind of "based on" you're interested in:
* Officially authorized works: These are few, mostly limited to adaptations like the "Chronicles of Narnia" film series.
* Unofficial sequels/prequels: There are several, but they are not considered part of the official canon.
* Fanfiction: The number is vast and constantly growing, making it impossible to count.
Let me know what type of "based on" you're looking for, and I can provide a more specific answer!