Spider-Man is arguably the most popular Marvel character. He was the top costume among boys this Halloween with 2.6 million dressing as their beloved web-slinger. Because of the popularity of the character and how long he’s been around, many different variants have been created. To keep these different Spider-Men, women, monkey, pig, ghost, alien, mutant, and armored characters from clashing, Marvel has wisely put many of them in completely separate realities/universes.
Artist Dell’Otto created a banner with 38 of them, which I believe is all of them. Imgur user Mmmasian has created a gallery that breaks down every single one of them. This is an incredible resource and something you’ve gotta dedicate sometime to reading.