I think Batman easily tops the Phantom. In the Nolan Trilogy Imposters are only mentioned shortly, but there's also the whole "I am Batman" movement going an - I don't think they ever mention the exact numbers or anything unfortunately.
Also, if you expand superhero and make it being with superhuman feats, then the Master from Doctor Who might qualify as well. He created like 7 billion copies of himself in The End of Time (called those new humans the Master Race). It's only hinted at, but he most likely transformed dead bodies as well, making the number even bigger. It's not obvious whether they just share appearance or abilities as well though. They don't really take over his role though, also he considers them all being "us", himself included.