Welcome to BYC!
As you say you are already planning on many more birds I would definitely suggest that you build for what you will have, not what you do have - take the maximum number you *think* you might want, add a few more, and then use that for your building plan. The coops section mentioned above is a great place to start getting some ideas - and from there you can tweak it to what you want. The "needs" of the birds are pretty basic - enough space and a well-ventilated, draft free area in which to be sheltered from the elements of the climate you live in - - from there all the bells and whistles really boil down to personal preference and what is most convenient for the flock keeper. ......
If you have an idea of the breed(s) you want to add, remember to factor that into your plans as some breeds will have different needs when it comes to coop design.