My mother-in-law is one of eight children, each of whom has produced two or more children of their own. That, plus their (multiple, for the four who've had one or two divorces ) spouses, and their children's spouses, drives the numbers up fast. And then there's the fact that my wife's generation has started spawning, so there are yet more children being added. Then you toss in the various adoptees, and things go straight to hell. Plus there are still about five people from the grandparents' generation still kicking around and throwing another straw on the pile. We draw people up from North Carolina, Florida, and (now that my wife and I live there) Ohio. In theory the family unit is based in Illinois, but they just kind of exploded to the four winds and multiplied like rabbits in the Australian Outback. And they're so clannish that anybody who can even briefly be accounted a member gets party invitations for the rest of their days.
My own family is similar but can really only muster up to the high thirties or low forties when the big reunions swing around. Fortunately they're also crazy-localized and would riot if they got invitations out to Ohio. Reduces our workload quite a bit. Also fortunately, we're just having my wife's parents, her two brothers, and the brother's girlfriend that everybody knows is going to be the next in-law. Immediate family only on this one, and my own genetic accidents now live out West and cannot attend (woo!). My sister-in-law would be here as well, but is studying in Italy. So it'll be a seven-person meal for our first hosted Thanksgiving. Much more doable than the clan-wide doomstacks.
My father-in-law, meanwhile, is an only child. Go figure.
The guest obligations are sort of similar with our families, but the host really only asks for people to bring the small stuff (wine, chips, dip, etc.), or something they simply can't make for lack of equipment or experience. I know that's not even a regional thing so much as how it expressed in our specific family units, though, so I guess that's just a highly-varied standard.