A lot of people wonder why we just don't throw in 30 levels into the pack, and I have an opinion on that. The overall attention to detail paid to a level at id is just incredible. The designers go through the design and construction phase, after which it is play tested until we are all sick of it. Then the artists tend to take over and design custom textures, art, shaders, effects etc for the level. Then the designers go back through, doing final tweaking and cementing out the item placement. It is a long process.

There hasnt been any certified quote from the id guys on exactly how many levels are in the pack, although it has been mentioned off-handedly a couple of times that there more than the originally quoted 12. We'll see soon enough eh?