We usually recommend you allow an absolute minimum of 2 square metres of run space per bird, Christian, and more is better especially if they are to live in the enclosed run all the time rather than free ranging. So with 42 square yards ( 35 sq.metres) you could plan for up to 15 -16 birds max. But 12 would be better.
Its always good to give them plenty of space as they will fight and get stressed and peck feathers out if overcrowded. Also, with a good-sized run and quite a few chickens there will be times when you need to divide the run, eg when introducing new birds, so bear this in mind. Possibly consider two smaller coops rather than one big one, or even two smaller flocks within a divided run.
If this is your first try at keeping chickens you might like to start small and build up your flock gradually, as you get more experience and maybe your tastes and interests in different breeds develop. If you can get to the point where you have a range of ages in the flock they won't all get middle aged and stop laying at the same time. Instead you'll have a more steady supply of eggs throughout the year.