My friends and I were talking about this. I came up with Johnny Depp:

Hunter S. Thompson
James Barrie
George Jung
Donnie Brasco
Ed Wood

some upcoming roles in which Johnny portrays real people:
Elle France editor Jean-Dominique Bauby in "The Diving Bell and the Butterfly"
John Wilmot a.k.a. the Earl of Rochester in "The Libertine"

previous movie:
Scotland Yard Inspector Fred Abberline in "From Hell"

not sure if you'd wanna count this one, it's another one in production in which he plays "Lindsay" in "Shantaram" in which he plays the role that's based on the life story of the author of the novel, Gregory David Roberts.

Dustin Hoffman - Lenny, Little Big Man, Finding Neverland, All The President's Men, Billy Bathgate, Papillon, The Messenger.

Probably Charlton Heston who has portrayed (among others):

Sir Thomas Moore
Henry VIII
Cardinal Richelieu
Robert Devereaux (Earl of Essex)
Michelangelo
John The Baptist
Thomas Jefferson
Buffalo Bill Cody
Pres. Andrew Jackson

and that Moses guy.

Paul Muni might be up there with Heston too, I'm not sure. Or maybe not.

Gary Oldman has played:

Joe Orton
John Richie (aka "Sid Vicious")
Dracula
Lee Harvey Oswald
Ludwig von Beethoven
Pontius Pilate

Dracula wasn't a real person. Vlad the Impaler was who Dracula is based on but Dracula was not.

But Gary did look like Vlad, though.

I can only come up with 6 for these two:

Henry Fonda

Frank James
Abe Lincoln
Douglas MacArthur
Thomas Watson
Wyatt Earp
Teddy Roosevelt Jr.

Rod Steiger:

Napoleon
Al Capone
Pontius Pilate
W.C. Fields
Mussolini
Col. Owen Stuart

Jason Robards (including his TV work):

George S. Kaufman
Abraham Lincoln (twice)
Al Capone
Doc Holliday
Brutus
Gen. Walter Short
Lew Wallace
Ben Bradlee
Dashiell Hammett
FDR
Howard Hughes
U.S. Grant
Andrei Sakharov
Henry Drummond
Armand Hammer
Mark Twain
Alvin Binder

I thought he played Vlad in the movie, but whatever...

A good list, but...

Was Jack Crabb in Little Big Man fictional or factual? (I thought the character was entirely fictional.)

And, in The Messenger, wasn't he just an apparition that Joan of Arc saw while imprisoned, so he didn't actually play a real person (?)

I thought Little Big Man was a true story but based on the person's embellished tales. And I never saw The Messenger I just remember him doing an interview for it so I figured he played a real person.

You know, it's probably one of those guys from a HIstory Channel docudrama. You know - they all dress up, everything's slo-mo, and you never get t hear them actually talk. They probaly use the same guys over and over.

"So, Jim, who do you have this week?"
"Well, Monday I'm playing Heinrich Himmler... tuesday, I'm Anne Boleyn's executioner..."

Edward Herrmann
Joseph Breen
Raymond Massey
William Randolph Hearst
Branch Rickey
Nelson Rockefeller
Fred Friendly
Alger Hiss
FDR (once)
George Bernard Shaw
Max Eastman
Lou Gehrig
Franklin D. Roosevelt (twice)
Franklin Delano Roosevelt (thrice)
and Warden Peter Soffel (who I think was a real guy)

Larry King has played "himself" 41 times.

So did he played him 5 times, 3 times, or 2 times?

Damn, I was gonna say that one. If cameos count, then Larry King will win this one by a mile.

I don't think Larry King counts because he plays the same guy every time, so it's not the most real people. It's not even people, just one person. It's not as if we're going to count Ian Holm's multiple portrayals of Napoleon as separate people.

If TV counted, it would have to be someone from Saturday Night Live-probably Phil Hartman.

Here's a partial list of Sir John Gielguld's credits:

Elizabeth (1998) .... The Pope
"A TV Dante" (1989) (mini) TV Series .... Virgil
A Man for All Seasons (1988) (TV) .... Cardinal Wolsey
"Wagner" (1983) (mini) TV Series .... Pfistermeister
The Scarlet and the Black (1983) (TV) (as Sir John Gielgud) .... Pope Pius XII
Gandhi (1982) .... Lord Irwin, Viceroy (Edward F.L. Wood)
"Marco Polo" (1982) (mini) TV Series .... The Doge
Inside the Third Reich (1982) (TV) .... Albert Speer Sr.
Chariots of Fire (1981) (as Sir John Gielgud) .... Master of Trinity
The Elephant Man (1980) .... Carr Gomm
Caligola (1979) .... Nerva
"Edward the King" (1975) (mini) TV Series .... Benjamin Disraeli
Julius Caesar (1970) .... Julius Caesar
Oh! What a Lovely War (1969) .... Count Leopold Von Berchtold
St. Joan (1968) (TV) .... The Inquisitor
The Charge of the Light Brigade (1968) .... Lord Raglan aka Fitzroy James Henry Somerset
From Chekhov with Love (1968) (TV) .... Anton Chekhov
Campanadas a medianoche (1965) .... Henry IV
Becket (1964) .... King Louis VII
Saint Joan (1957) .... Warwick
The Barretts of Wimpole Street (1957) .... Edward Moulton-Barrett
Richard III (1955) .... George, Duke of Clarence
Julius Caesar (1953) .... Cassius
The Prime Minister (1941) .... Benjamin 'Dizzy' Disraeli, aka Lord Beaconsfield

Depending on the source anyway There's a lot of speculation that Vlad wasn't really that big of an influence on Bram's book. The legends of where the stories come from are about as widespread as all the Dracula adaptations by now.

Gary Oldman did play Vlad the Impaler in the movie, if you recall the flashbacks.

A few for Spencer Tracy. There might be more, but I'm much too lazy to check out every one of his credits.

Judgment at Nuremberg (1961) .... Chief Judge Dan Haywood
Inherit the Wind (1960) .... Henry Drummond
Thirty Seconds Over Tokyo (1944) .... Gen. James Doolittle
Edison, the Man (1940) .... Thomas A. Edison
Stanley and Livingstone (1939) .... Henry M. Stanley
Boys Town (1938) .... Father Edward J. Flanagan