To serve the nation's health-care needs and be cost-effective, at least half of all medical school graduates should become primary-care physicians, say health reformers. In contrast to the nation's 125 medical schools, which on average produce only one primary-care physician for every five who earn medical degrees, the UC Davis School of Medicine already exceeds the goal. Sixty-six percent of those who graduated this year chose primary-care residencies. And alumni statistics show that 46 percent of the 1,820 who received medical degrees at UC Davis from 1972 to 1992 trained in primary care, with 75 percent now practicing in California.