Run size

We are new to keeping chickens and have a few questions regarding run design and number of chickens it would be healthy to keep in it.

The run is 12ft x 8ft max.  We have a coop big enough for 5/6 chickens but will be possibly only getting 4 depending upon advice received.  For info the coop is on 18" legs so they can use the full area of the run.

The run will be 6ft high with 1/2 inch galvanized mesh on the lower half plus a 1ft skirt.  The upper half and top will be 1" or 2" galvanized mesh, again depending upon advice received.  The roof will be be partially or completely covered with sloping corrugated plastic.

I also plan to cover the whole floor area with 1/2" mesh (straight on top of the grass) then have 2 areas, one with straw and the other with woodchips or bark.

The chickens will be let out in to the garden for an hour or so daily but the most important question is, if they could never be let out of the run, how many chickens would it be healthy to keep in it.

Sorry for the long-winded post and thank you in advance for any/all replies.   

Like Shez says, 4 would be okay in that area.  I wouldn't put more in.  DEFRA recommends 1 sq meter run space per bird, but we try to get people to double that AT EAST if they can!

If you think, the ancestors of chickens used to live in the jungle and had miles of space and trees to run and fly around in.  Seems cruel to coop them up in tiny spaces like some people do.