Game Masters Guide, Brawls and Mobs, page 68-89. The mob swarm total HP is increased by 25 percent. Is this 25% based on the HP of one creature, or the total of creatures? In the same paragraph it says a Mob Swarm is comprised of roughly 50 creatures. A commoner has 8 HP. Does this mean that 50 commoners in a mob swarm only have 10 HP (8+25%) or would they have 8x50+25%? And are they still dealing only one attack with no increase in damage?
#GMG -Brawls and Mobs
gaunt pulsar
gleaming wind
The swarm uses the base statistics of the creature and increases the HP by 25%.
In your example, a swarm of angry commoners would have 10 HP, and it would have all the same actions as a standard commoner. For the commoner specifically, I could see a GM ruling that Angry Mob would deal ~51 damage (49 members + the starting member who deals 2) since all of the commoners in the swarm are within 10 feet of each other.
The swarm mechanic is intended to be a quick/on-the-fly way for the GM to manage a large group. It replaces the need for the GM to run 50 stat blocks at once, but it isn't intended to be the sum of 50 stat blocks together. It allows for the epic "mowing down hordes of goblins with each swing" scene without adding too much extra bookkeeping for the GM.
gaunt pulsar
I understand, but I still think if it has 50 individuals in the mob it should be a little tougher in both HP and damage. It would have been nice if the Angry Mob mechanic from the Commoner stat block were applied to mob swarms in general in the GMG.