@commuterface
My family never does this as there aren't enough of us but I do know many families who would have 20-30 people for Christmas lunch. Extended family can multiply very quickly. One of my friends will have 4 generations for Christmas lunch this year. I don't understand why you think this is so alien?
@buzzkillington
We are hosting this year (20 in total) and I know guests will come laden with champagne, wine, port and cheese as they always do. This is more than enough of a contribution imo.
@mindutopia
My dh comes from a family in his mum's generation of 5 children. 5 aunts and uncles and their partners alone is 10 people, plus a grandparents is a few more, then each of the 5 siblings has between 3 and 4 children each, so in our generation there are 18 cousins, plus their spouses, and now several of us have a kid or two. 10 aunts/uncles + 2 grandparents + 18 cousins + 18 partners of cousins + 6 small children is 54 people. And that's not even including my mum and stepdad or other extended family or the lonely elderly neighbour and there's always a few of those too.
Sorry I wasn't going to post again, but I HAVE to know this.
How? 
How can the people, claiming to have 25 to 45 people for Christmas lunch, POSSIBLY accommodate so many people, and COOK the Christmas dinner for so many people in a single kitchen with a single oven and a single 4-pot hob? (Which is what the vast majority of the population have.)
HOW?
I simply cannot get my head around the logistics of having 25-45 people for Christmas lunch. The most that most people could fit in would be perhaps 8. Or 10 maximum. I have cooked dinner for 9 people recently (including me and DH) and it was really stressful!
What kind of kitchen do these people have? Do they have a 30 x 40 foot kitchen in a mansion, with servants and cooks and bottle washers? And half a dozen 10 kilo turkeys??? 
How the hell do you find all the crockery and cutlery and pots and pans? Or do your 'guests' bring their own?! Do you put on the ticket charging £30, 'must bring your own plates and cutlery?' Coz who the hell has enough plates and cutlery for 25-45 people?! ?!!!
What about the seating? Where the hell do they all SIT? 
They must have a dining room the size of a bleedin' tennis court!
This puts me in mind of those daft adverts where the stressed and hassled 'mum' is cooking Christmas dinner for 25+ people, when in reality, having 25+ people for Christmas dinner rarely happens to anyone.
@ReanimatedGB
I wonder how much of the whining about this is also bound up in the idea that women exist to service others, for 'love', and that they should never expect any kind of reward.
The only one to mention that is you.
@Willow2017
To expect your mum to cater for all that every sodding year without contributing a thing is greedy and lazy. None of the family bring anything to help feed them all. Maybe in your family they do but not in this poor woman's case it must have cost her a fortune every year.
Whose fault is that? No-one ASKED her to do it. The whole story (in the media) sounds like bullshit to me. Like much of the claims from people who say they host Christmas dinner for 2 to 4 dozen people, and have a £1000 grocery bill for Christmas dinner alone!
And I re-iterate, I don't know a single soul in real life who does this. Only on mumsnet do these outlandish claims appear. There are some massive stealth boasts on this thread. Yuk. Very unpleasant to see.