There is no fixed number that works for every article, but the way readers feel when they go through your post should guide you. If someone reads your article and feels like every sentence is pushing them to buy something, they will leave and not come back. A good way to think about it is this: links should appear where they make sense, like when you mention a product you are actually talking about. One or two well-placed links in a post usually do more than six random ones. How many links do you currently use per post?