how many chickens will fit

Probably about ten I'd say.

 

The standard ratio for large fowl chickens is one per 4 square feet of coop, and 10 square feet of run. So you could have 6-8 chickens comfortably. If you have smaller chickens, you could have a few more.

 

I have 8 girls in my tractor that is 4x8. The run is about the same size as yours. They seem happy and have enough room to spread out. On the few days I have had to lock them in, there have been no problems as there is enough space for the lowest hen to get away from the dominate ones. I freerange my girls (supervised) when I am home so they get a little more exersize and goodies. Cant wait for all this snow to melt so they can freerange again!!!

 

I agree that 8 would be the max in a coop/run that size. I have the same set up but my girls freerange all day long. They only come into the run during the day to "nap" on the saw horses I have set up in there and the coop to lay eggs. When I have to lock them for any reason they do fine, although they don't really like being locked in! It will probably help if you have things for them to do to keep them somewhat active and entertained, like a dirt bath.
CJ

 

Remember that a long enough roosting board is also required. I find 10" per bird is about right. 8' roost means 10 full sized birds. They like being close in the cold, but want more space if it is hot. Understandable.

 

8 or 9 at most

 

The run is a little small for 8; I wouldn't get more than 6, unless they will be let out of the run daily.