Don't get caught in this situation.
Groups of zombies, known as mobs, swarms, hordes, drags, appetites or plagues are often an overwhelming and awe-inspiring force, consuming everything in its path. These groups can be split into several different classifications:
The Pack[]
Threat level: Moderate
A small group of ghouls made up of roughly six or less. Generally, there are several guidelines as to whether a group of survivors should commit to killing zombies if they are not yet under attack. In general, the undead should never be underestimated, and engaging even a small number of them should be done as safely, quickly, and overwhelmingly as possible.
Accuracy in reporting their numbers is also very important. One slip up on a head count can have dreadful and irreversable consequences. Two should be a duo. Three, a trio. Quartet, Quintet, Pack of six, Pack of seven... all are more specific terms less likely to lead to erroneous planning.
The Horde[]
Threat level: High
Generally more than six or seven (which would be a small horde). The difference between a large horde of thousands and a chainswarm is mostly semantical. Also, the term is usually reserved for a group that is actively pursuing targets to consume.
The Herd[]
Threat level: Very High
Hordes that are both passive (or merely without a target for the moment) and traveling in the same direction - almost as a migratory flock of birds, or cattle. Herds can become aggresive hordes quickly as soon as one finds a target and notifies the others with an excited groan. Stealth is key.
Chainswarms[]
Threat level: Extreme
The worst case scenario of what is already a worst case scenario. There is no precise number of zombies that comprises a chainswarm. Needless to say it is a very large attacking horde. So large, in fact, that one cannot tell where the boundary of the group ends - it stretches past the horizon, or the immediate field of view. Chainswarms deplete an area inhabited by the dead by at least 90%, as their domino effect of moaning for reinforcements drains towns and fields, and sometimes entire states, provinces, or small nations dry of zombies who can answer the call.
Without a well prepared platoon of organized veteran fighters and a reheased plan of defense (including days, perhaps weeks worth of supplies), there simply is no way for survivors to defeat a chainswarm. The only option is escape, and even this may prove difficult for two reasons. First, since by definition, one cannot see where the line of zombies ends, there is no guarantee you're not surrounded, as reinforcements flood into the chainswarm from all sides. Second, even if one retreats to a structure the dead can't enter, like a fortress or on top of a mall, if one zombie has seen where you went, the area will continue to be surrounded indefintely, and death by lack of water or food may be imminent.