> If there was a constant inflow of flies into the room, after some time you would have an infinite number of flies. With a constant finite inflow, you’d only

> If there was a constant inflow of flies into the room, after some time you would have an infinite number of flies. With a constant finite inflow, you’d only have infinitely many after infinite time. As to why they don’t keep amassing: flies need food, water (moisture), warmth and oxygen. Stuff too many in a room, and conditions elsewhere will be better for them.