Welcome! First you can never have too big a coop. Chicken math always sets in. As others have mentioned, if you start small and pick a breed or two you like and want to try, or get one each of several breeds, you will find there is that certain breed that just grabs you. Or you will find there is a breed you really really want to try and you haven't done so yet. That way you have enough room to add more birds of your choice without figuring how to expand or enlarge the coop or even add a second coop. (All things we've done.) mumble mumble next year I want to add....And on it goes.......