If they are important, use one of the NPCs from the back of the MM. If they are not important use the Commoner's stats.
But sometimes it doesn't matter how much hp a resident of a town has. If an event is scripted, stick with the scripting. In my game, a doppelganger assassin was trying to from the party for the murder of a well loved innkeeper. The assassin gets 1d8 piercing + 7d8 poison + 4d8 piercing if it surprises a target. Additionally, if the target is surprised, it gets advantage to attack and any hit is a critical hit. That is 24d6 of damage, but the innkeeper is supposed to be bleeding out and the PCs should have a chance to save him. So in this case, hp didn't matter. I just narrated passed a note to one of the player's to describe in his own words that he came downstairs from his room called the innkeeper over, stabbed him and ran back up the stairs. Most of the PCs were still downstairs, and discussing things among themselves, so they didn't notice the note passing. Their dumbfounded looks when the player stated his character stabbed the innkeeper in the chest were great. Thankfully, the Paladin leaped over the bar and healed the innkeeper. If I worried about hp here, the innkeeper would have been dead outright even though he was a retired adventurer.
For your town of adventurers, just grab various NPCs from the back of the MM.
