Currently the maximum number if website editors is 10. Generally speaking, there is rarely a need for more. If, for instance, you were publishing a blog with many conrtibutors, authors can submit posts to a website editor through other means and the editor can approve and publish the post. This makes for better site security as well.
There are also solutions that can help automate these kinds of processes; if your needs fall into this category, we can provide you a quote for designing and implementing an appropriate workflow.