When a silverfish is damaged, if the damage killed it, it will not cause other silverfish to spawn.
Linked issues
duplicates 1
Comments 4
In any case it has been like this since I first started playing Minecraft (1.2.5). I am unable to find a related bug report so far, so we may have to check with the developers (though this is rather similar to MC-2432).
Working as intended. See wiki.