I think you are worried about the wrong things Milo. I see from your other questions you are planning on writing the game is C++.
Even a not very well coded sever written in a scripting language would be able to handle 100's if not 1000's of requests with out apparent lag in a card game.
These kinds of technical details are probably not going to be a big problem. If they do become a problem it would only be a day or two of optimization or moving things around a little. But only after the game is finished and only after you have identified what exactly the problem is..
Writing a headless client now to simulate lots of uses is a waste of time.. focus on the really hard problems like..
- actually finishing your game and getting it online and
- finding the 100's of people who want to play your game simultaneously.