It's a kind of odd coop -- I get the impression that the wire-floored back 'shelf' was what the chickens 'roosted' on? Not the usual arrangement.
If they will be ONLY in there, rather than free-ranging most of the day every day, I would not personally put more than 3-4 in there unless you built a much larger run (since closing in the 'downstairs' would only give you 4x8 of run space). If you did build a larger run, perhaps 6x10 plus the closed-in underneath, you could get 8 or more chickens in there but I would suggest erring on the side of fewer rather than more. Or if they will free-range all day every day you could just go according to however much roost length you can get in there.
How to install normal roosts would depend somewhat on how many chickens you intend to put in there.
You'll probably want to close off most of one or two sides for wintertime, although you *might* be ok with one side remaining open in winter depending on what your winter lows and winter storms are like.
Good luck, have fun,
Pat