The following error message displays when opening a map document that is already open on 18 or more computers:
"Failed to Open Document."
This is a known limitation.
Solution or Workaround
If the map document needs to be opened on 18 or more computers, or in 18 or more instances of ArcMap, create copies of the map document and give the map documents different names so that the maximum number of instances per map document is not reached.