There are 19 for Steam on Windows/Mac OS (15(?) for XBox Live Arcade, different exclusives).
The Wikipedia page has a list of which characters are available on which platforms, and their origin.
To quote:
These characters can be unlocked by collecting enough bandages or completing certain warp zones.
There is a fairly solid list of how to get pretty much everything here.
Summarised:
Character Origin # Bandages
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Head Crab Half-Life 10
Josef (Robot) Machinarium 30
8Bit Meat Boy 40
Naiji Aquaria 50
4Bit Meat Boy 60
RunMan RunMan Race Around the World 70
4Color Meat Boy 80
Captain Veridian VVVVVV 90
Steve (Mr. Minecraft) Minecraft 100
Character Origin Warp Zone
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Commander Video BIT.TRIP RUNNER 1-12
Jill Mighty Jill-Off 2-8
Ogmo Jumper 3-16
Flywrench Flywrench 4-18
The Kid I Want to be the Guy 5-7
Character Available
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Bandage Girl Once you beat the main game, only in The Cotton Alley world
Meat Ninja Once you 100% the game
Character Character Select code Activate by selecting
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Super Tofu Boy petaphile Meat Boy
Goo Ball ballgoo ???
Brownie RB,RB,RB,B,B,B,X Meat Boy
To unlock on the Character Select screen, just type in the listed code on the selection screen which appears when choosing a stage.
Super Tofu Boy was put in following the PETA incident.
According to this, activating Brownie on the Character Select screen requires an XBox 360 controller.
Teh Internets Chapter is also unlocked by collecting bandages (20x) and is an online chapter which contains bonus levels.
Best Answer
Mana's guess is correct. I can confirm that after beating Level -1 (in The Forest) I got the N#7*<1!23 achievement. Six worlds, six negative levels, six achievements.
See this question on how to reach the levels in the first place.