Three.
The tradition began with the current Queen's grandfather, King George V, in 1932.
When he died, his eldest son, King Edward VIII, did not deliver a Christmas message due to his abdication of the throne.
Edward's younger brother, King George VI, resumed the practice (albeit sporadically at first).
After George's death, his daughter, Queen Elizabeth II, who is the current Queen of England, continued the tradition set by her father.
So in total, that gives us three monarchs who have delivered a Royal Christmas Message:
1) George V
2) George VI
3) Elizabeth II
