There is strength in numbers, and you’ll find this phrase often repeated simply because it’s true. However, numbers become less efficient if there is no organization involved. Each member of a group must have synergy with other members, so they make the most use of their capabilities instead of just standing there or even hindering each other.
In an open area, the most important key to facing off multiple opponents is to keep your distance and don’t allow them to circle you. Flanking is a common tactic since wars in ancient times, and it still applies to the modern era. Getting blind sided is an automatic loss.