In SharePoint 2013 OnPremises the maximum number of authors in an Office Document is configurable as It’s stated in the following TechNet Url: http://technet.microsoft.com/en-us/library/ff718248(v=office.15).aspx. Therefore, we could understand that there isn’t a limit regarding to this setting…and we would wrong. Fortunately, in the SharePoint Software boundaries and limits article we can find a partial answer to this question (http://technet.microsoft.com/en-us/library/cc262787.aspx):
The recommend co-authors number in a single Word or PowerPoint document is 10.
And the maximum number of co-authors in a single Word or PowerPoint document is 99. It’s not possible to overpass that limit since it’s a threshold.
However, I’m not 100 % sure if these limits are also applicable to other Office products where co-authoring is also possible: http://technet.microsoft.com/en-us/library/ff718249(v=office.15).aspx
Finally, let me share with you a PowerShell script that allows to read and update this setting for a given Web Application in a SharePoint 2013 farm:How to read and modify the maximum number of co-authors for Office documents