My husband and I have always loved hosting Thanksgiving. Even when we were in a teeny-tiny apartment that did not technically have a dining room, we were happy to jam in our family and friends and crowd around the table in (what we thought was) a convivial, Bohemian way. One year we didn't have enough chairs so we used a love seat as a bench, which semi-effective. (Except only the tallest guests could sit there, and it was so comfy my brother-in-law fell asleep during dessert!)
Which leads me to the one thing all Thanksgiving hosts should check right now: How many chairs you have!
Dining tables offer you a certain amount of flexibility on how many you can seat at them — we've been known to squeeze 12 around a table for six (it was just a little tight around the elbows). But chairs aren't that great for sharing. Two people in one chair isn't cozy — it's uncomfortable, especially for a long meal. And unlike tables (which you can build with some plywood and cinderblocks in a pinch), chairs are surprisingly hard to fake. Your end tables are too tall, your ottomans are too short, and standing isn't really an option, either!
So take the time now to do inventory through your home. Count every side chair, low stool, office chair, etc. Do you have something for each person on your RSVP list? If you don't, think about investing in folding chairs. IKEA's most basic ones are only $8 and you can stash them in a closet for your next event. (IKEA's next tier up ($20) might actually even look cute hung from your walls!) You could also consider a folding bench, because a bench offers more flexibility for how many you can squeeze on them. I now have a folding outdoor picnic bench (alas, discontinued) from World Market that has served us well.
If you don't want to buy something (or don't have enough storage space for even a few folding chairs), considering borrowing! Start asking your neighbors now if they have chairs they can spare for the big event. Even if you only get one or two per neighbor, all your guests' bums should be covered in no time!
Do this. Now. Seriously. Because it's one thing to run out on Thanksgiving Day for an extra stick of butter, but it's another thing to have to run out in a panicked search for chairs.