You gotta keep moving against a group. Preserve your stamina. Wait for one of them to become isolated. Get your whacks in where you can. This is how you beat multiple opponents by yourself.

More generally, focus on footwork. Learn to step into and out of enemy melee range just long enough to provoke a strike that will miss, leaving you an opening to attack safely. Only spend stamina to dodge as a last resort. Always leave some stamina in reserve specifically for this.