Depending of your OkiOki license, 25, 70 or 200 documents per month are included as a standard. When you go above that number then there is a cost per document that will be invoiced. But there is no physical limit. You can manage as many documents in OkiOki as you'd like.
However, if your accountant offers you OkiOki, there is never a limit on the number of documents.