The problem with answering this question is that it's very difficult to know what would have happened if some person did not exist. Some reasoning among similar lines can be found here.

This section from 80,000 hours career guide might also be relevant. I should note that a recent EA forum post questions whether Stanislav Petrov made the difference that 80,000 hours assume here. Some other people who plausibly saved many lives include Louis Pasteur, Maurice Hilleman, and Norman Borlaug. But increasing human population has many indirect effects and it's very difficult to evaluate them.