Total HMO enrollment grew 6.1 percent, to 76.7 million, during the last six months of 1997, according to InterStudy, which tracks HMO trends.... Enrollment in PPOs is soaring. The American Association of Health Plans says PPOs have added 44 million members in the last five years, while HMOs have attracted 30 million.... How many doctors are enough? A Harvard School of Public Health survey yields differing opinions. Fifty-five percent of HMO executives said there is a general oversupply of physicians; 70 percent thought there are too many specialists. Six in ten specialists disagreed.... American Osteopathic Association delegates approved a resolution that D.O.s who provide primary care and osteopathic manipulation should get higher capitations than M.D.s.