Are you planning on keeping food or water in the coop? That might take up more space. Also, how much roosting space do you have?
Good questions - roost space is the most limiting factor.
That said, for the sake of peace, I'd also advise to just stick with 3 to start. IF you decide to get more birds later on (which you'd need to do to sustain egg production over time) then expand the entire set up at that point.
It says EEs are small birds. 4-5 pounds for females. So many things from this book arent matching to real life and I just don't want to do this wrongs.
Your book isn't wrong necessarily. Problem with sizing standards for EEs is, depending on the hatchery (and sometimes even the year or season) is that they can have different parentage, so sometimes they're small and svelte, other times they're heavier. My 2 current EEs are noticeably more "streamlined" compared to my other birds. I had another EE before that that was same size as other standard birds.