Hey man, welcome aboard, even if you are an Eagles fan.
I honestly think that four backs will make the roster; Portis, Betts, Rock, and Nemo. Betts has been fantastic in every game and Rock and Nemo have just played too well in the preseason. We need Nemo for short yardage because he is just a wrecking ball but it's going to be hard to let Rock go just in case of injury.
All the guys have looked good catching the ball out of the backfield and I saw Cartwright even lined up as a WR in one of the games so he has some versatility (not many people know this but even though Cartwright is a little guy he had the best 40 time AND bench press of all RBs at the combine when he came out). I think he'll make it but he'll probably be inactive for quite a few games unless there is an injury or some other problem.
A couple of times we have used two backs in the backfield with Manuel White lined up as a FB so maybe Nemo may do this in the regular season but he has pretty much exclusively played RB in the preseason.
Actually, it's a little hard to guess at these things just because our backs are asked to do a lot of different things. Sellers and White will probably be the guys who would most likely line up in a FB position.
Those guys are both really H-backs though. I think that White is going to be put on the practice squad, he is having to learn the position from scratch.

ey and Sellers will be the H-backs.
So sorry if this is a bit rambling but I see four RBs and two H-backs making the final cut.
Other than Portis and our QB,

ey is THE most important part of our offense. He has hardly been used in the preseason and this shows me that Gibbs has lots of plays drawn up for him and he doesn't want to show his hand. What's more,

ey and Ramsey have a great connection. He will put up big numbers and he is the guy who will be setting the stage on all the Gibbs misdirection plays. Also, I firmly believe that Gibbs has so much confidence in

ey that that is one of the reasons he decided that he could do without big recievers. Also, look out for Robert Royal in red zone situations. Last year he only really played in the last six weeks of the season and he scored four TDs in the last five games. he only had eight catches for the year but four TDs! He is another big red zone target but he is a true TE in Gibbs' system and therefore he is not nearly as important than the H-backs.
"We're not going to be the pushovers of the NFL, we're gonna push over some people!" - Clinton Portis