According to this Wiki page, the models go like this:
Has it ever been explained what happened to model Number Seven?
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(Most of this information is from the battlestar wiki for Number Seven, quoted here for completeness)
Yes, in one episode Ellen explains what happened to the Sevens.
The original seventh model was named "Daniel", and was described as very human-like (artistic, sensitive, etc.) Ellen, in particular, seemed to consider Daniel her favorite "child". Cavil became jealous of that relationship and destroyed the emerging Seven clones (he did something to the nutrients that were used to grow the clones) and killed the original Daniel.
With no original and no clones left the entire Seven line was dead. It was never shown on-screen and, apart from a handful of mentions in an historical context, plays no real part in the show. (Other than to show how much of a sociopath Cavil is).
In particular, many fan theories tried to put Seven as either Kara's father, or Kara herself, but Word of God was pretty explicit that Daniel never appeared on-screen.
The simple truth is that Daniel was meant to be a model, but never part of the series. From an interview with Ron Moore, BSG creator
Q: How about fan theories over the show, like mention of a missing Cylon named Daniel? After his name was mentioned, the fans just went wild online. Did you intend for that to happen, and was he supposed to inspire this big fan-driven backstory?
A: You know, the Daniel thing is going to be one of the great fiascos of the show, in terms of what fans thought and what the truth was. Because Daniel was not intended to be anything more than an interesting bit of backstory in that episode. And that's how we approached it. It was just a story that Cavil and Ellen tell each other, that sort of goes to the idea of who Cavil was and how deep his resentments were, and his jealous nature - and [we wanted to] do a Cain and Abel allegory. That was all it was.
And then after the show aired. I started picking up all this stuff about how fans were obsessing about Daniel and how [people thought] Daniel was Kara's father, and he was the big surprise. I started thinking, "Oh shit, slow down people, I don't want you to really get invested."
I usually don't like to go out there and say, "Oh, that's a bad theory," because part of the enjoyment of watching the show is coming up with ideas. But this was gathering such momentum, I didn't want people to be going into the finale and really be waiting for the Daniel shoe to drop, when there's no shoe. It's one of those things where you're inside the show, [and] you look at it, and go one way. And then it's broadcast, and an audience sees it, and then they seize on this piece that you never really anticipated, and then you're sort of amazed. And you're saying, "Slow down, no - come back."
"Daniel" was referred to as one of the final cylons. Daniel Graystone is that "Daniel". He made his appearance a generation before the rest at re-begat the cylon race.