I can't remember the terms exactly in relation to a population for events but a lot of it comes down to what I'll call steady flow population IE those who go for weakly service or whatever vs flux population that being those who go for events or whatever. Just using a portion of my previous example if a building is rated for say 200 people and 300 people show up it's a shit show waiting to happen, that's not getting into the possibility of road blockage and outflow issues that could persist for an extended period of time and interfere with things like ambulances.

Also pretty sure there are effectively standing permits that can be obtained for concerts and shit. Frankly speaking this type of shit varries city by city county by county, so some places will have far more stringent laws and bylaws for such things while others won't give an iota of a fuck so long as they call ahead of time so a cop can be present in case of a shit show.

Though I will state that I am thoughly biased against using churches in particular for such events because a lot of them are built in just the dumbest way possible. I got a good sample size when I was a kid in boy scouts where I got lost in the backrooms of a church on several occasions looking for the restroom. Also a lot of them are just in really weird areas with a questionable amount of parking I remember there was one where we were doing a charity dinner and the sheriff showed up because there were too many people and they were blocking the road.