Make it two. I have 2 at the moment with no problems. But before I had 3 and sometimes the resourses were overstretched. I opened a second account for the 3rd and am adding another soon. But it depends on lots of factors. The other plugins you use, cache plugin, images optimized or not, the products and number of images and on and on. You'll just have to experiment.
@Mar has several thousand products in one store and I think she has a second one.
Like she said above and similar to what I just said, it depends on the content. You may be able to run 3 stores on the same account with little or no problems. I did, but with rare problems.Every setup is different.