1) Create a new empty mission
2) Place a unit and a zeus game master module, link the unit to zeus
3) Start the mission and enter Zeus mode
4) Choose 'Groups' and the NATO 'Sentry' group (the specific group doesn't matter, but Sentries have a small number of groups)
5) Start placing sentry groups (hold CTRL to make this faster)
6) Continue placing sentry groups (ensuring they remain SEPERATE when you place them) until the game stops allow you to place units
At some point (100 or so groups) Zeus will no longer spawn units when you attempt to make a new group. After this point, even deleting existing groups will not allow you to place new units.
If you switch to another side you will again be able to place units for that side (up to the limit).