According to KFC, you can get "10 pieces any way you want it," which generally means you can choose from Original Recipe, Extra Crispy, Grilled, or Boneless (I say "generally" because with a franchise model, there tends to be inconsistencies).
With 10 pieces, you generally get two or three breasts. If you want any more than that, most locations will charge you extra.
The deal is for a limited time at participating KFC restaurants through November 24, 2013.
Here's a 15 second TV spot for the new $10 Weekend Bucket: