You're correct that there's only one livery per aircraft when the sim is installed, but I believe there's some new ones that are supposed to show up on the marketplace (for free) sometime soon. As I understand it, Asobo made some changes with the texture files that still aren't completely documented in the SDK (and haven't been puzzled out by repainters yet), so I'm sure we'll get some repaints in the near future.
Also, Asobo has said they're trying to work with airlines to get some officially licensed liveries (they've previewed United and Iberia liveries for the A320), so I wouldn't be surprised if more show up as those agreements get hammered out.