The rule of thumb that I've read elsewhere, is a square foot inside a coop for each bird, and a square metre in an outside run.
If you pace it out, it looks kinda small, so you may want to add some more.
Currently, I have 4 birds in a run 3m by 2m, so 6 square metres, which I think is enough. There is also a water container and a feed container in with them.
They do also get regular time running around in the garden, so they're not cooped up in there all of the time - but they could be without a problem.
Inside the coop, the important bit is how long the perch is. Mine is about 1m long, or so, which allows all four to perch happily together. I've had fewer bird in the same space, and they all just cwtch up together anyway.