A swarm is considered a ~5 creatures of a specific creature.
For instance the players provoke the town guard, approx 5 guards show up to fight the PCs. A swarm of guards is created for the encounter which all act on the same initiative count. Instead of running 5 individual guards, therefore simplifying the encounter for more "theater of the mind" improvised encounters, and help the GM.
V Dice Multiplier (× v_dice when computing V from dice): 5×