In order to answer you question it would be helpful to know who "The World" refers to. Are you talking about a U.N. body, individual nations or some other global body? Global population is an interesting and complex issue. While global population is growing, some populations in Europe, Italy for example, are at zero or sub-zero growth. Who decides a limit and would it be by region or country, and who enforces the limit? Intimately tied to this issue is poverty and race, as well as increase longevity. Perhaps your question could be turned around to ask what is the maximum age that a person can live.