"Many. The four that come quickest to mind left roles for very different reasons:
Dean Jones, COMPANY
Madeline Kahn, ON THE TWENTIETH CENTURY
Judy Kuhn, SHE LOVES ME
Bette Davis, NIGHT OF THE IGUANA"
Because despite the thread title, many people will assume these actors were all fired (I mean, few actors leave a Broadway show willingly), let me clarify three of the above:
Dean Jones was going through his own divorce during Company and was an emotional wreck. He asked Hal Prince to let him out of his contract. Prince agreed provided that Jones opened the show.
Madeline Kahn was more or less fired from On the Twentieth Century, but it was through some sort of "mutual agreement" with Harold Prince to leave the production. She did not enjoy being in the show and I think wanted to leave.
Judy Kuhn was not fired from She Loves Me, she just didn't do the commercial transfer and had unfortunately left by the time the cast recording was made.