MEXICO CITY — HOW many people live here?The preliminary results of the 1990 census peg the metropolitan area at 15 million inhabitants or 18 percent of the nation's total. Many experts put the figure closer to 18 million. "If you compare this figure to the 1980 census, it is hard to accept," says Manuel Ordorica Mellado, a demographer at the Colegio de Mexico. "We haven't been able to closely analyze the data yet." City officials are more than ready to accept the latest census. "We have some breathing space. The trends are with us," says Mayor Manuel Camacho Solis. "The rate of migration into the city is dropping." There has been a migration from the city center to nearby, smaller cities. Because of pollution, high costs, and crime, "the migration rate has dropped," says Crecencio Ruiz Chiapetto, professor of urban planning at Colegio de Mexico. Mexican officials have tried for more than a decade with little success to "decentralize," to put the brakes on the city's growth by stimulating growth elsewhere. It appears the boom in factories along the United States border in the last five years has helped. City planner Jorge Gamboa de Buen says that to reduce the growth rate further the nation must continue to decentralize industry, reduce the birthrate (the national goal is 1 percent by the year 2000), and eliminate city subsidies for public services. Urban planner Ruiz Chiapetto says the inertia built into the growth patterns means zero population growth isn't likely until 2020 or 2030. By that time, Mexico City will have a population of 25 million or so - a conservative estimate, he says.