There's an arbitrary limit on how many units you can control, usually exists to counter game engine's poor performance with larger numbers or to counter poor network code.
Alternate names: Population limit, Unit cap
The first video game about Unit limit was released on August 25, 1997.
Commonly the limit is 250 or near it. Most common real reason for this is that transmitting movement and other info of so many units or more real-time over the internet for multiplayer games is slow, especially in games with more than two players as each player can control up to that many units, so for 4 players you're sending info of activities of 1000 units and so on.
Note that this does not include cases where availability of minions is limited.