I don't have room for everybody!
If you run out of room at the table, set up an auxiliary eating area (kitchen counters work in a pinch, but living room coffee tables and den game tables are good options). Who sits there? It can be done by age, by birthday month, by favorite color, or simply grab the couple of people you were most looking forward to seeing and sit there with them. In any case, Thanksgiving is all about spending time together―if you keep that in mind, everything else will follow.