As a rough guide, I try to have an AP cover a radius of 20m for a good result. So if I was looking at this for indicative pricing, I would tell the client you can get an "ok" result with 2 x APs (most areas covered well but expect some areas where it won't operate so good), or a great result with 3 x APs (saturation coverage).
Because of the cost of a site survey and because of the small area - it is going to be most economic to just put in more APs.