Actually, remember that I said that they live as a parasitic adult for 12 to 20 months. That's when they actually kill fish by parasitism. Then they go into tributaries of the Great Lakes to spawn. Once they have spawned, they die. Any given female can deposit anywhere from 25,000 to 100,000 eggs.
These fish are not an exception, in the sense that, just as with any other fish, most of those eggs don't actually survive. But up to about 6,000 of those eggs per individual can survive and do survive, on average. So one individual could potentially give rise to up to 6,000 other lampreys.