With later studio multitrack recorders, one guy could sing all the parts on a cut (& even play all the instruments), laying down each in turn on a different track on the master tape (these went from 2 track, 6 track, 12 track, 32 track, etc as the equipment improved) ... BUT .. Edwin was talking about getting the start to his singing career (back in Cleveland) in the late 50's / early 60's. At that time he wasn't a solo lead singer but a member of his first harmony group (the Futuretones ... name inspired by that of Sonny's group, the Metrotones) . He gave me the same info when I interviewed him at the Jazz Café many years ago & also credited Sonny Turner for 'schooling' him & his fellow group members. He was talking about the period before he went into the army. At that time even the best most modern of studios only had 2 track machines. Of course you could 'overdub' on the same track, but you lost much of the strength (volume) of earlier efforts each time you went back & taped on top of the previous work.
My article on Edwin's Cleveland Years was published in a fanzine at the time but it has since been added on here. Just do a search of the Soul Articles section to see it all ... EDWIN STARR'S CLEVELAND YEARS.