I'd write four of them -- one to each of my grandparents.
Three of them would basically ask for background information. My maternal grandfather (who was in an AGR with my grandma) died several decades ago. My father's parents died when I was an infant.
As for my maternal grandma, she survived until I was a teenager. It was hard to watch her succumb to her illnesses, so I'd mostly apologize for being awkward and let her know that I truly loved her (and still do). I'd also thank her for keeping a dark family secret away from the light.