If you shut them in at night, they need enough space to move around when they wake up in the morning, before you let them out. That is even more true if you sometimes lock them in during the daytime (examples would include bad weather, or if the outdoor run needs repairs, or many other possible reasons.)
A general guideline is 4 square foot of space per chicken inside the coop. That is meant to be enough if they are sometimes shut in while they are awake. For 11 chickens, that would call for a coop of 44 square feet.
If the chickens are never shut in the coop, and they simply go in to sleep and come back out as soon as they wake up, then they only need enough space to sleep at night, plus a bit more space to shift around as they are getting settled. That assumes that you never have weather bad enough to keep them inside. In that case, you can best judge by looking at how much space they have.
Even if you do have enough space for more, adding new chickens might cause trouble because the new ones and the current ones will probably not want to sleep near each other at first. So that is another detail you might need to consider.
I really can't say what will or will not work for you. I'm just trying to explain some of the ways people try to figure this out, and some of the things that need to be considered. Of course it is ultimately your decision to make.
