Ok here goes:
I do 22 people and LOVE it.
I do cook absolutely everything apart from desserts wich are brought by my best friend and i do a soup, starter and the xmas dinner has turkey and ham.
I get tables and chairs from the people that are coming, so my mum gives me extra chairs and my friend has extra foldable tables, it stretches along the kitchen through to the hall, you want to be able to sit everyone at a table. I personally dont think couch seating etc is as important as people will sit at the table and between rooms as the day goes on an dinner is finished.
I have a hostess trolley, passed down to me and its a life saver but the fact that everyone is doing a dish is fab!!!
Set up your table on xmas eve or the 23rd, cutlery and everything all on the table - i know it sounds simple but one year i forgot to set the table and it was carnage.
Use disposable tablecloths, amazon or tesco have good decent plastic type ones that wont disintegrate with a splash of water and so when the tables cleared of dishes you can lift and bin everything else.
I also do a table plan with wee place cards again just makes things a bit less chaotic so everyone takes their seat and there is no faffing bout when your serving.
You can NEVER have enough tea towels and loads or Christmas paper napkins on the table.
Have everyone arrive early so you can see what dishes you have and what needs to be heated up etc.
I hope you have a dishwasher, each course set it to the fastest possible wash to help with clear up.
We do clear and turn around the house when everyone leaves, yes we can be up till 3am doing it but walking down on boxing day to a house thats back to normal is an amazing feeling, then i just spend the rest of my day in jammies eating leftovers.
Honestly its me favorite day of the year, yes its busy but my kids love it and so does everyone else, as long as you have loads of food and drink and a place to sit everyone will be delighted.