By our count, there are 14 alternate costumes or characters you may play as in Batman: Arkham City, through a variety of promotions, pre-order bonuses, and timed-exclusive downloadable content available now. The "good" news, I suppose, is that all of this stuff will be available at some point—for a price.
So if you're intent on collecting Batman's entire wardrobe, don't run out and buy the NOS energy drink, the Green Lantern DVD, and two copies of the game, one from Best Buy and the other from GameStop. Unless, of course, you really want to.
Of course, the bad news is that all of this stuff will be available at a price. Specifically, $5 will get you the Skins Pack on Dec. 6; $7 gets you Nightwing on Nov. 1; and another $7 gets you Robin's DLC on Nov. 22. Catwoman is included with new copies of the game, and GameStop is even giving out Catwoman DLC codes for used copies. Otherwise it's $10, sort of like an Online Pass.
There was also a pre-order bonus that did not include any skins. The Joker's Carnival, a challenge map, was offered by GameStop (U.S.) and Tesco (U.K.) No word on its availability otherwise. The Iceberg Lounge challenge map was included in the $99 Collector's Edition. No word on its future availability.