According to this Wiki page, the models go like this:
Has it ever been explained what happened to model Number Seven?
(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.