And I think it should be at least 4. I also read somewhere that 4 was for small birds like Leghorns and for big ones like Orpingtons it should really be 6 or something. So even the number based on breed can vary a lot too. I don't think anyone really uses 2 except maybe getting into more commerical ones. But obviously not full commercial cause they just stuff birds in cages. But still. 2 is pretty small I think. Most things I've read say 4 sq ft inside the coop and 10 sq ft outside in the run but since yours free range that's not really applicable. But you also have to remember that they are in there all night. I see a lot of people say they will "only" be in there at night, and I used to be guilty of it too, till I saw someone say night's a long time. Another reason to make sure you have adequate ventilation. That said, I think your 10 will be fine in that coop especially if they get to free range but I would not add any more. They could get by but they'll probably be suffering. Example: when I first got my chicks we bought a little premade coop from
TSC, I think maybe 2x3. We soon realized the run was useless so we took it off (though the base under the coop wouldn't come off, it held the coop up) and put it inside a 6x12 dog kennel we happened to have. Once it was safe, they moved out to it at 7 weeks old. Well, it was fine for when they were little, but they were in that thing for 2.5 years. They quickly outgrew it. And even the run wasn't really big enough since it was basically split in half by the coop especially with under the coop being fenced in so it created two little aisles along the fence. Eventjally they started sleeping on top of the coop and I let them (we fenced the top) except in bad weather or cold. We didn't mean for them to be in it so long but we took a while building it ha we started March 2016 (got the chicks October 2015) but then we stopped working on it for a while, started again, etc. And my dad also had other stuff to work on. Anyway, we finally finished it and they just moved in at the end of April. So they were in that thing way too long and they weren't happy. And of course the ground was completely barren and gross by then and hardpacked. I had tried various materials that all just blended together eventually. And it got so muddy with rains. They are much happier in the new coop and the plain dirt drains much faster though I am adding pine pellets soon cause it's getting a little messy to dust bathe. The run could be a little bigger, still 6x12 so I plan on expanding it, BUT there is no coop in the middle anymore so it feels bigger to them lol the coop is 4x8 and I wouldn't add more, even though I really want to. But I did lose one this last fall to a hawk

so I only have 7 now. I just realized she never got to experience the new coop and feel bad she never got the chance
That said, I wanted to build bigger and my dad thought even this size was a chicken palace and also that it can hold more than 8. I know it can't or at least not many more so I wish we had built bigger. Although it is a palace compared to their old coop lol I could probably add a couple more, we have plenty of roost space. I put in two roosts the 4 foot way directly above the nest boxes but they choose to roost in the rafters. Except for a couple I guess lower ranking ones. So younger chicks could sleep on the actual roosts. But I don't want to overcrowd. Mine rarely free range but I'm starting to more. We've had a few predator scares.