there's a lot, tho imo not enough
all champions have one skin, with most having 2 or more and a few having 5 or 6. there's a bunch of legendary skins of varied qualities (some are godly, some are really bad) and there's 2 ultimate skins, both of which are pretty awesome but also pretty expensive (tho discounted enough on release that a lot of people get them)
there should be more tho, i want more
MORE
Ah cool. Not all heroes have cosmetics yet, but to steal a post just now in the Dota thread.
Wow, the amount of heroes with no hats at all is a lot lower than I thought
Ancient Apparition
Bane
Elder Titan
Enigma
Io
Jakiro
Magnus
Night Stalker
Phoenix
Shadow Fiend
Timbersaw
Tinker
Treant Protector
Troll Warlord
Undying
Venomancer
Viper
Visage
Zeus
It's basically just an odd assortment of heroes either already set up for items and not workshop ready (Treant, Tinker, Night Stalker, Troll, Undying, Magnus), heroes that are due for a remake (SF, Zeus, Jakiro), or a few oddballs like Phoenix who aren't going to be remade but aren't set up for items (also AA and Venomancer). Oh, and Io.
Seems like of the heroes that can have items, have items (except Elder Titan). Although counting Terrorblade might be cheating, as he's not item ready yet, but has one cosmetic.
(Thanks Ballad!) As you can see, Valve make them ready and then people can make items and submit them for approval. Recently there has been some grumbling about how little information on how to get something approved there is, but I honestly don't know much about it. That said, the level of 'marketing' these sets get seems to keep getting better and better. Take this set for instance. A creative trailer with lore for the set, all made by the community. It's not better than riot or anything, but I wanted to point it out because it's just nice to see.
Edit: I see I was beaten to the Witch doctor set video haha, we all love that one.