Probably none,maybe if you are lucky 1. Mosquitoes chances are wouldn't be attracted to a fly trap because its not giving off co2, which attracts mosquitoes. Flytraps have tiny hairs that need to be triggered twice to make the trap go off. read more
But if you think of it in this perspective, if each person killed one, that would be 7 billion. Plus there are reptiles and other such animals that eat them. Add to that those killed by insecticides. read more