Yeah this is still my issue (I'm only like 5 hours in).
I can deal with most of these guys 1v1, but I can't manage crowds at all you lock onto one person and just get stabbed in the back, and if you don't lock on then good luck landing attacks
with time you'll get the hang of dealing with it.
different enemies try to swarm in different ways. for example, with those little goblin mobs with wooden shields in east limgrave, one will actively try to circle around you and smack you while the others occupy you head on. but if you run parallel with the runner, he'll end up running away from their group, right to you, and you can fukk him up quickly, before the others arrive.
humans tend to rush straight in, so AOE weapons or attacks with wide range tend to help with crowd control.
another tactic: if you have a bow, use it to snipe dudes from distance. even if it doesn't kill them, the person you hit will come to you to fight. usually without alerting everyone else. this is always a go-to when dealing with giants, stormhawks or those burger king magicians. my weapons are currently strong as fukk, but i dont have spells that give them AOE effects yet, so i tend to potshot and 1:1 whenever possible.
if you dont have a bow, you can also learn enemy patrol routes, and creep into a solo enemy's sight-lines in ways that make them walk over to check you out. as long as they dont have a horn (and no one else sees you) they'll eventually just come over to try and kill you.