What are the server and internet connection specifications?
There are, and always have been, limits to units and AI the software can push. The hardware you run on is something you should also factor into the equation.
For example, our server can only handle about 600 static objects before they start vanishing. That's with any number of players from 1 to 10.
From our testing, we think fewer large groups is better than many small ones. In other words 5 groups with 10 units is less taxing on the server than 50 groups of 1 unit.
On our server we tend to start getting more disconnects above 12 clients. We know some servers out there are hosting 30+ clients.
There are many variables to consider.