qaz123tfg:
Haven't noticed you before, so just in case...
...Welcome to SD!
Crashing as I believe your question conveys, doesn't relate to units on the map in that way.
I'm sure there is an amount that can crash a scenario, still the game will most likely become unplayable due to lag, long before any crash occurs.
Most of us are interested in how many units can move on the map without creating lag. That figure will vary from system to system.
I suggest you use the search feature and search for old topics about crashing and lag.
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