This is funny. Apparently, somone actually calculated how many people you need for a multigenerational space trip :
“For a space trip of 200 years, perhaps eight to 10 generations, his calculations suggest a minimum number of 160 people are needed to maintain a stable population. This would produce around 10 potential marriage partners per person, he says, and if this seems a small number, “think about how many people you dated before you got married”.
They talk about some other stuff, including my favorite genetic subject, inbreeding. [link spotted at Arstechnica]