Hi,
I think you should try to there is no official comprehensive list from Hasbro detailing the exact number of unique characters released in the Marvel Legends line. However various fan sites and forums keep track of these figures. According to some of these sources, there have been around 400-500 unique characters released in the Marvel Legends line since its inception. Despite this Hasbro often revisits popular characters with new sculpts, accessories, or paint jobs, which can lead to multiple versions of the same character being released. For a precise count and updates, checking dedicated collector websites or forums like TheFwoosh or Marvelous News can be helpful as they frequently update their databases with new releases.
Thanks