Chickens require 4-5 sg.feet per bird not counting roosts and nest boxes. In the outdoor run they should have 10 sg.feet per bird. When chickens are overcrowded they can resort to fighting, feather pecking and in extreme cases cannibalism.
If you are getting a pre-made coop they often inflate the number of birds that can comfortably fit inside.
Glad you joined the flock! Have a look here for coop designs and ideas and build it a big bigger than planned, just in case… (I also started with 4 chickens, a year later I had over 100)