The National Planning Assn. said the Los Angeles metropolitan area will have the most new jobs between now and the year 2000, followed by the Boston metro area. There are expected to be just over 26 million new jobs available for Americans by 2000, the study says. The Planning Assn. study anticipates that Los Angeles will add just over 1 million new jobs for a total of 5.3 million in 2000. But the association, an independent economic research group, said Boston will add nearly 755,000 new jobs for the second-largest growth, followed by Anaheim, San Jose and Phoenix.