I think of ‘mass’ in the context of defining groups of things as just ‘a lot under the circumstances’. A mass gathering for an NFL game is 100,000 people; a mass crowd for a high-school JV basketball game is probably 100; a mass crowd for a 1 year old’s birthday is maybe 50. It’s relative to what is expected under normal circumstances. 4 people being shot or injured is a lot because nobody should be shot or injured.