The standard amount of revisions per artwork proof is 3. This will depend largely on the type of design involved. If it's a standard print order 3 is usually enough. For logotype designs sometimes 3-5 revisions is sufficient as well. Nevertheless, sometimes clients add products to their orders after seeing the first proofs, and ideally each additional product will include 1 revision per item. So, in short - each product proof includes one (1) revision and up to two (2) additional revisions for corrections and editing until the product is satisfactory and approved for production.
Sometimes clients have new print projects in mind that would entail further graphic design work. That may include one or several revision until done right. That's also why each project has an artwork proof associated with it. It is only in the cases where artwork revisions take more than the average amount of time for the same type of project that higher artwork fees would apply.
Usually, the project manager in charged of your project will let you know how many revisions will be included in your order.