I tried running a few tests yesterday on what happens when you have a lot of units and here are the results :
Once you have more than 2000-2100 units, some of them start disappearing off the minimap, older ones first.
Once you have too many units (haven't been able to determine a number, but it's above a few thousands), the newly created units cannot be given orders and don't have an attack.
Unit creation may block if there is no available point within a radius of the selected point.
The maximum amount of units that can be on the map at the same time is 16383, or 2^14 -1. If you can get above that, please contact me.
I tried running a few tests yesterday on what happens when you have a lot of units and here are the results :