Nope, it's all behind the curtains.
The shadow's ability says "Create Spawn (Su): Any humanoid reduced to Strength 0 by a shadow becomes a shadow under the control of its killer within 1d4 rounds."
So it is limited to humanoid targets and since it's supernatural, one presumes it's a controlled ability. I.e., a shadow chooses to use this ability or not.
Assuming that's the case, there's a good chance Shadows have developed an instinctual breeding habit to avoid overpopulation. Or perhaps they develop some sort of tribal structure where they actually need permission of a higher ranking Shadow to create spawn, or have rules they follow before they create spawn. Of course, being always chaotic evil makes that scenario unlikely.
Another possibility is they tend to get killed off rather easily. Mid-level wizards, clerics, paladins, bards and so forth have little trouble with Shadows. Shadows are only INT 6, so they ain't tactical geniuses.
Assuming no weird Shadow customs and going strictly by what's in the Monster Manual, PHB and DMG, what probably happens is Shadows start off solitary, created by a Create Undead spell cast by an evil cleric of 12th level or above spending one hour's effort and 150gp for the black onyx used in the material components. The cleric then commands undead to control it. Since he can only control a number of hit dice of undead up to his his level, the safe thing to do is command them to not create spawn once he's at his limit of about 4 shadows. An insane chaotic evil cleric might not impose this restriction, but he's risking them getting out of control.
Anyway, assuming a "free range" shadow is running about, it'll continue making spawn until it reaches the "swarm" level of 11 or so shadows. By then, some high-level party will get wind of it and kill off the shadows pretty easily.
The other option is for a shadowdancer of 3rd level to have his shadow companions make more shadows, but the shadowdancer has no way to control shadows, so it's safe to assume he orders his shadows to make no spawn.
Hope that helps,
Greg