When outnumbered, just take what your opponents give you. As mentioned, generally people become much more careless when they have numbers behind them and make mistakes such as holding a chambered attack while closing in on you, while you're backpedaling. Most doing this won't suspect a sudden lunging attack on your part, and you can score a hit or kill with good timing. The weave/dance thing works well too, if the enemy is being reckless with their swings.
In the reverse situation, when you are one of many bearing down on a lone enemy, you want to be patient and try to surround them. As room is tight between you and your allies, you need to either deliver thrusting or overhead attacks and be precise with them.