On our first trip, 20. The second trip, 60. A lot of our character greetings were at character meals, those are easy to add up because you know which characters are there. Always got Mickey and Minnie at MK, and the Pooh characters at Crystal Palace restaurant. At Epcot, there's the Epcot Character Spot with Mickey, Minnie, Goofy, Pluto, Donald. Animal Kingdom had Flick, Pocahontas, Baloo, King Louie, Rafiki, Timon, and a few others. Hollywood Studios also had a lot - Mickey, Up Characters, Monsters, Power Rangers, Incredibles, Green Army Man...... I probably forgot a few. We also had some characters sign tshirts with their character on it, instead of the autograph book.
And we hardly ever waited in line, at least not for very long. Although I did see some long lines in MK. But we also went at a non-busy time of year.