You assert "plenty of defenses against torpedoes don't work against [UUV]". Based on what? What is this hypothetical property of a UUV that is superior to a torpedo?
A UUV with sufficient range and warhead is going to be big and heavy. Long-range torpedos weigh 2 tons each for a good reason. Calling something a "drone" or "UUV" does not imbue it with magic physics. It still has to cross some long span of water with enough speed and a large enough warhead and a guidance package capable of finding the target.
What kind of vessel are you going to use to bring these UUV within range of the target? 20 torpedos would be almost the entire magazine depth of an attack submarine. Surface combat ships carry even fewer.
You seem to be ignoring all evidence from how modern naval systems actually work when discussing your hypothetical UUVs.