So that means roughly 12 characters per FE game, if we ignore 11 and 12 as unique games.
Contrary to what some people are thinking. I don't believe anyone can marry anyone, a la FE 10 support system. Let's pretend for a moment that there are 120 people who can marry anyone. (it's doubtful in my mind that DLC can marry, but assuming 'some can' plus the number of playable characters native to 13, it's conceivable it'd even out) 120 x 119 = 14,280 unique conversations. I'm betting it's pre arranged who you can marry.
Anyone can marry, but not everyone can marry anyone.