It kind of makes all negative connotations of the word fall away if that's the case. You could roll out anticipating a guerilla strike again 30 guys using 10, get attacked by 2, and be a "zerg" if you're determining it by the scale of any enemy force you happen across in the open world.
For smaller scale stuff, around x3 the numbers is a zerg, examples;
1v3, 2v6, 3v9 etc
For larger scale stuff, more so 2x -x2.5 is zerging, examples;
10v20/25, 20v40/50 etc