(3KillerBs has helped me a bunch, so I'm stealing their format!)
For each adult, standard-sized hen you need:
- 4 square feet in the coop (.37 square meters)
- 10 square feet in the run (.93 square meters),
- 1 linear foot of roost (.3 meters),
- 1/4 of a nest box,
- And 1 square foot (.09 square meters) of permanent, 24/7/365 ventilation, preferably located over the birds' heads when they're sitting on the roost.
Your coop is 6x4 (24 sq ft), which is enough for 6 chickens. Your run is 12x6 (72 sq ft), which is enough for 7 chickens. You have 12 ft of roosting space, which is enough for 12 chickens. You need to use your limiting factor to determine your maximum, which in this case is your coop. Therefore, your maximum is 6 chickens.
As the other posters said, that setup will be overcrowded with 10 chickens. It may seem okay right now, but as they grow you'll likely run into issues.
How big is your yard and where are you from? The first idea that comes to mind is that you could turn your current run into a big coop, then predator-proof part of your yard to make it a big run. If you had a 12x6 (72 sq ft) coop and the proper amount of run space, you could have up to 18 chickens!
