There are a large number of adaptations of Robin Hood, with (I think) at least 40 being films. Speaking of full-length feature films only (no TV, shorts, etc), is the Robin Hood myth the most adapted story in history, or is there one with more?
Here is a link to Wikipedia's list of movies and TV shows featuring Robin Hood.
I think that there are fewer than 50 theatrical movies on the list.
I have often read that Sherlock Holmes is the most often filmed character.
It has been estimated that Sherlock Holmes is the most prolific screen character in the history of cinema.
Redmond, Christopher (1993). A Sherlock Holmes Handbook. Toronto: Dundern Press. p. 164. ISBN 978-1-55488-446-9.
Sherlock Holmes is the most portrayed literary character in stage, film, and television history, having appeared on stage and screen 254 times as of 2012.
I think that the list lists 77 Sherlock Holmes theatrical movies.
So I am inclined to believe that there are more Sherlock Holmes theatrical movies than Robin Hood theatrical movies.