The most I had was 16 for Thanksgiving 2 years ago. We host every year, but it's normally about 8-9 people...and every family brings at least one dish with them. We just cook the main dish and a few sides, and provide the pies and rolls, etc.

But our diningroom is set up perpendicular to the living room, so we were able to set two dinging room tables, a card table, and the outside plastic table up with tablecloths and there was room for everyone, extending into the livingroom.

If your backyard were bigger I would suggest some plastic folding tables and chairs, and some heaters...and doing it outdoors. *pout*