A little more information would help. Where are you located so we have an idea of your climate? Putting your general location in your profile can help with this type of question.
Size and layout of the coop would help a lot. Maybe a link to a website with photos inside and out and enough info so we know which one we should look at if they have more than one.
How much outdoor space do you consider adequate? How is that outdoor space set up, will it be covered in snow part of the year. How do you manage them, lock them up in the coop section each night or leave the pop door open? How you manage them is quite important.
You might follow the link in my signature to see some of the things I consider important concerning space. I’ve found that the tighter I house them the more behavioral problems I have to deal with, the harder I have to work, and the less flexibility I have to address issues. A lot of that is more about your comfort, convenience, and stress levels than the chickens.
Is that a current photo of the chickens? They look quite young. That 4’ roost will probably still be adequate for five chickens, even when they are fully grown. Trying to put even one more chicken in there might work, but the roost will be getting pretty crowded. Based on roost length alone I’d say you are at your limit without more modifications. It’s not just how tightly they squeeze together once they are on the roost, they need room to get on the roost. In that coop it may be they just step on it, but if they have to jump up at all, they will probably spread their wings, which requires room.
I’d guess that has at least three nests, which is way more than just adequate, it’s overkill. That won’t hurt anything but it’s just more than you need. Nests are not a limited factor for you.
If you leave the pop door open 24/7 and the weather and set-up is such that they can actually go outside every day of the year if they want to, coop size becomes much less important behaviorally. Chickens don’t differentiate between coop space and run space. They just need enough space whenever they need it. It doesn’t matter if it is inside or outside as long as they can use it. When it is dark isn’t a big issue, as long as that coop stays dark all night, street lights or security lights might cause a problem in keeping it dark. If they are locked in that space after they wake up you could easily have issues. Are you willing to go out there at the break of day every day of the year and let them out, even when you have the flu or really would like to sleep in on a Saturday morning?
If that is all the coop you have integrating more chickens is likely to be pretty rough. Integration is one time you need a lot of extra space.
The tighter you pack your chickens the more you have to manage the poop. That’s part of working harder. Chicken keeping should be a pleasure, not a chore.
Where will your feeder and waterer go? If you add another roost can you keep the feeder and waterer in there without then pooping in them from the roost?
There are several things I don’t know about your specific set-up or about that coop, but it looks a lot like others we’ve seen on here. In my opinion it is not even close to adequate for 6 to 8 chickens. You are pushing the limits with five. In my opinion, if you try to add even one more you are setting yourself up for a lot of stress and taking a risk for creating serious behavioral issues among the chickens, even if you are successful integrating. Instead of asking how many can I possibly shoehorn into this tiny space, look at it as how can I give my chickens adequate space.
Edited to add:
I see you and Mary posted while I was typing. 3x4 is not adequate for any more chickens. You are really pushing the limits now. You might be OK in a decent climate so they can get outside often, but that is tight.
Edited by Ridgerunner - 12/15/16 at 6:36am