Of course, nothing is more fun than tossing multiple knives at once towards a target. However, while homebrewed enemies might be able to fire a knife swarm at the party, you won’t be able to do the same without doing some homebrew yourself. Otherwise, your character will just need to content themselves with being able to upcast the spell. You could say that the reason the cold damage goes up on an upcast is that the ice knife itself is much bigger and explodes into more shards, but that’s probably as close to the official version of multiple knives as you are going to get.