[ahartmetz]

From experience: about 5x as many as you think

Story: I was living in a large flatshare that was plagued by flies from nearby horse stables. We were organizing a decent sized party (maybe 40 guests). In the afternoon, we had 15ish items of food that needed to be stored for a few hours until guests arrived. At first we considered covering all the food (creating work and waste of aluminum or plastic foil), then I had the idea to kill all the flies in that room and close the door. We saw about four flies. In the end we killed about 20 before we declared the situation good enough. I don't recall if there was a last fly that we eventually killed, or that we eventually gave up on killing. It took half an hour or so.