Hmm... the mention of all the Robins has me wondering which single superhero identity has had been used by the largest number of people. I think it may be Captain Marvel (the Marvel-Universe version, not even counting Billy Batson). Apparently seven people
have borne the Captain Marvel name at least briefly in the Marvel Universe. There have been only five official Robins per current continuity, plus Carrie Kelly in an alternate future, plus the young Bruce Wayne according to the pre-Crisis continuity of the '50s. That's also seven, but only five within the same reality, so I'd say CM comes out ahead.
Oh, but wait a minute -- apparently there have been eleven bearers of the name Starman
within DC history, although one was a villain named Star-Man so I wouldn't count that. Not sure how many of those are in the same continuity, though.