Welcome!
Having your approximate location on your profile helps a lot; climate matters when building a coop and run. The size recommendations given above, and ventilation, all really are important minimums to use when setting up for your flock.
Your photos show a cute red building, very small, with no ventilation or light inside. It's great to have that concrete foundation, but if that's your run, it's also cute but totally not predator proof, and if you are in snow country, not built for snow load on that tarped roof.
Twelve hens will do fine with four nest boxes, a minor point in all this.
Over time, our flock grew, and our coop/ run combination was enlarged and upgraded. What you have now may hold three or four hens, if you add ventilation to that coop, and lock them in every night. Much better would be to build a safe roofed run, so they could be in it 24/7, giving them enough space and safety.
Mary