In theory, since most of the characters are optional in the World of Ruin, there should be a number of different endings after you defeat Kefka.

Has anyone gotten them all? How different are they?

There aren't different endings per se, just mini endings for each character. Four of them can't be missed (Celes, Edgar, Setzer and Terra), the rest are optional but three change depending on who you have (Relm, Celes and Edgar), and two can't be accessed at all without getting different characters before hand (Umaro and Strago), and one person doesn't have their own mini-ending (Sabin). This means there are 16 possible "endings".