I'm pretty sure I cant name 19. Do Mohamad Ali and Cassius Clay count as 2?
Let's see...
Joe Louis
Max Snelling (probably spelled that wrong)
Ali
Tyson
Holmes
Was Marciano a heavyweight?
Foreman
Norton
The two Russian brothers... Klitschko? Is that their name?
Leon Spinks
Michael Spinks
Buster Douglas
Frazier
I'm gonna mix my list with yours and see how close we get:
Schmeling
Baer
Braddock
Louis
Dempsey
Marciano
Patterson
Liston
Ali
Frazier
Norton
Holmes
Spinks
Spinks
Tyson
Douglas
Holyfield
Klitschko
Klitschko
Wilder
If we're sure that both of the Spinks and Klitschko brothers held the title at some point, that's 20. Only 8 to go.
If I rack my brain, I might be able to come up with one or two more, but I'm about at the end of my knowledge. The last 20 years are a dead zone for me. You helpfully reminded me of the Klitschkos, and I'm familiar with the current kid, Wilder.
Let's bold the 21 we've got. If any of these are wrong, we're screwed anyway.
Walcott
Schmeling
Baer
Braddock
Louis
Dempsey
Marciano
Patterson
Liston
Ali
Frazier
Norton
Holmes
Spinks
Spinks
Tyson
Douglas
Holyfield
Klitschko
Klitschko
Wilder