There are several aspects of this combat that are worth examining. The first is the nature of the combat itself. We have a situation where there are several men contending against a single man. We might expect that the several should be able to easily overpower the one. The description of the action, however, appears to have a single pair of combatants at any one time. Regardless of the protection of the Lord, it would be difficult for Ammon to be so fast that he could smite the arms of six men who attacked simultaneously. Even in the Asian martial arts demonstrations where one man withstands a number of assailants, there is typically only one pair in combat at a time, however rapidly they might come in succession.
This is partially cultural, and partially practical. The pragmatic reason for a single pair of combatants is clearly the danger to one's own companions from the swinging weapons. Both clubs and swords require room to make the swings, and friends might suffer as much as foe in a close encounter. In the ancient world, combat was typically very personal. The advantage of six on one was not the simple superiority of numbers, but the superiority of stamina. The effort expended by Ammon is clearly six to one. It is therefore not surprising that the larger numbers would typically be victorious. Even a temporary gain by the one might be assumed to be overcome by the fatigue of the effort.
The next question we have to work on is the nature of the weapons. We are given two categories, club and sword. For the clubs we have little information, but may suppose that there was no cutting edge. For Ammon's sword we certainly expect a cutting edge, but that makes the description of the use of that weapon curious. For some reason the text explicitly states that Ammon smote the arms of his enemy "with the edge of his sword."
If we return to our assumption that a sword in the Book of Mormon is a modern term for a more common ancient weapon such as the macuahuitl, then there is a possibility that this unusual description has a more logical explanation. The macuahuitl was essentially a club with embedded obsidian blades on two sides forming a plane. Thus the "sword" had two edges that were sharp, and two sides that were nothing more than a club. When used without deadly intent, the side might be used. The edges, however, were clearly deadly, and fit the description of using the edge of the "sword." The macuahuitl was not typically a stabbing weapon, but a slashing weapon.
The final question before us is the practicality of a blow severing a human limb. Without dealing with issues of anatomy, we need only reference the report of the Spanish that a blow of a macuahuitl severed a horse's head during the fighting of the conquest. Certainly a weapon capable of decapitating a horse was sufficient for the gruesome task described here (see the translation of the Titulo C'oyoi in Carmack, Robert M. Quichean Civilization. University of California Press, 1973, p. 303.