There are 12 human colonies.
There are 12 Cylon models.
But if you count Earth, there are actually 13 colonies. Maybe there are actually 13 models of Cylons instead of 12 with 2 that have not been revealed.
If they reveal the 12th Cylon sooner as opposed to later and is a relatively minor character like Tom Zarek or equivalent, I believe that it would be a decoy for the biggest surprise, the 13th Cylon that would be revealed in the finale. I believe that the 13th Cylon would then be a very major character like Starbuck, Admiral Adama, Baltar, or President Roslin and would happen near the very end. It would be a total surprise for all the characters and the audience because we're all thinking that once the 12th Cylon has been revealed that there are no more unknown "skin jobs" left. But, then we'd be wrong and surprised in the finale.
I know that the show states that there are 12 in the opening sequence, but this show is all about breaking conventions, so what's one more here?
According to Ronald D. Moore, Daniel is not a significant character.
"...I should probably say as a sidenote -- I know that there is a tremendous amount of speculation out there on the Internet -- that Kara is the daughter of Daniel or that Daniel, Daniel being the Cylon model that was killed or destroyed or aborted, however you want to choose to define it, by Cavil, that there's a connection between Daniel and Kara and that's part of the revelations that we're gonna reveal in the finale. I don't typically want to, like, put theories to rest, because it kinda spoils the enjoyment and fun of people in the show, however I do think it's worth saying that that is not part of the plan.UPDATE: 3/24/09
The 13th Cylon pretty much is a non-issue. I think Ronald D. Moore and the writers mentioned the 13th Cylon just to fix their screwed up numbering.