It's been done, so it's clearly possible.

One example: William MacBean. From his VC citation, "For distinguished personal bravery in killing eleven of the enemy with his own hand in the main breach of the Begum Bagh at Lucknow, on the 11th March, 1858." In the Indian Mutiny, he fought, single-handed, against 9 troopers, 1 naik (corporal) and 1 havildar (sergeant), and killed them all with his sword.

You can find similar feats on the battlefield in VC citations from the mid/late 19th century (and equivalents with firearms from then and later). Sometimes the VC winner faces a group in order to rescue a wounded soldier. Sometimes the VC winner doesn't kill them all - maybe only 1 or 2, and the rest flee, or merely holds them all off until help arrives. Sometimes the VC winner is wounded.

People also failed trying to do it. For example, the death of Francisco Pizarro. He was trying to defend himself against his assassins, and killed two of them. He then ran a 3rd opponent through with his sword (according to some versions, a 4th opponent pushed the 3rd onto his sword), and he was killed before he could free his sword. (And sometimes, the VC winners discussed above are killed.)

Such feats are exceptional - that's why they have resulted in the award of VCs, and the creation of legends and stories.

There are tactics and techniques that the lone swordsman can use against a group, and these tactics and techniques can be effective against relatively unskilled opponents. However, it isn't always possible to apply these on the battlefield, the opponents might not be sufficiently unskilled for such methods to result in victory, and bad luck (e.g., breaking one's sword) can happen. Plenty can go wrong, and one-against-many on the battlefield often results in the death of the one.

Can you please clarify what specific historical time or times and locations you are interested in? I may be able to answer your question, but as posed it isn't a historical one.