There is no general answer to this question. Also, from my point of view, you are choosing the wrong approach.
Given the system type, it looks that you are hosting on AWS. Which gives you a lot flexibility to choose the machines according to your requirements. You should first know your requirements, then start to build your application and when you reach a certain progress (not too early and not too late) you should start with performance tests. And then you can see if you need c4.2xlarge or smaller or bigger machines; if you need 2 or 4 or 10 of them.
Any other approach requires much more information which you probably don't have right now.
