We often have flies in our house, although we keep everything quite clean. We have the terrace door always open, if the weather is nice. Every day there are one or two flies flying about. But why aren’t there 10 or 15? If there is food , or if the house smells nice for flies, why don’t we get more? Occasionally I manage to kill them, but soon we have flies again, and never more two. It is as if they count the other flies in the room, and leave if there are too many.

Flies are solitary insects. So take for example a bee, when it finds something, it goes home to tell the others, flies don't. Thats why you're much less likely to encounter groups unless something really attracts them, like rotting meat.

How soon after you count two of them do you get rid of them? Maybe if you have it time, more would show up

Unless they're horny, in which case the horndog gonna follow everywhere.

Door is always open during the day; over all the summer months.