I run with 2 medium torpedo launchers (so in total 4 torpedoes) and it is usually enough. Sometimes 2 hits is enough, sometimes 4 hits is needed. I have never seen any ship survive 4 Reverberating Cascade torpedo hits. Tho i don't know what kind of ship i have attacked with them as we can't see what modules other players or npcs have or if they have modded them for extra/less health etc.
4 Reverberating Cascade hits is enough, getting 4 hits with 4 is other thing. Hitting npc with torpedoes is usually easy and they rarely avoid them, many players don't know how to avoid them or think 1 or 2 point defense will keep them 100% save, but many (pvp) players know how to counter them so some are hard to hit and some are able to avoid all easily.