I generally only do raids and gym battles during community / event days.
Salem is a very hot spot for Pokemon Go. Many people travel to Salem on community / event days because there are so many stops and gyms. Generally it's as easy as walking to the first gym when there is a raid event or on community days, a level 5 raid to find a large raiding party.
Our raid parties are always 2 groups of twenty. It it's not uncommon to end up at the same gym as an other large raid party. It's not unheard of to get 2 private lobbies full of blue, and 1 for red and 1 for yellow plus a public lobby at the same time.
EX raids, I have seen 200+ people show up to.
That being said, I know to avoid "The House of Seven Gables" gym whenever with a large group of people. There is a cranky old man who lives across the street who comes out whenever there is a large crowd that tells us to move along and calls the cops right away if we don't.
The House of Seven Gables is a famous house / a museum open for tours. It exist on a short dead end street on the water front. The guy who chases off all the pokemon players lives on the other side of the street across from The House of Seven Gables
The few times he has chased us off / was on the phone with the police while we did a raid were were all on the side walk on the opposite side of the street as him.
I rarely bother with battling gyms. Generally I just place pokemon in gyms with open space on my walk from the train station to work every day.