Surely this is a never ending chain though - four adult married siblings plus the parents of each spouse = five sets of grandparents/ parents of adults and four couples of your generation, so far 18 adults.
If you and each of your siblings has an average of two children, so far 8 children.
Then your husband is one of three, so his two siblings, their spouses and each of the spouses' set of parents. That's another 8 adults, and an average of two children for each of his siblings.
So far 26 adults and 10 children.
Unless your husband is the only spouse with siblings, there are still the siblings of your siblings spouses (if they're from smaller families one sibling each plus spouse is another 4 adults), so 30 adults, plus another 2 children per couple is 12 children.
Of course you can't exclude their parents-in law (another 4 adults, so 34 adults now), and the other children of the parents in law (if they each have 2 adult children another 4 adults, so 38 adults now), and the spouses of the other children of those in-laws (another two adults - up to 40 adults now - who have their own parents, so another four adults (44), but presumably they also have siblings, who have spouses (2 more adults), and the spouses have parents (four more adults) and siblings (2 more adults) and the siblings have spouses, who have parents and siblings...
It goes on exponentially. There's no cut off if all celebrate together.
That's without worrying about any stray great aunts and uncles who don't have spouses and children, or of course the grandparents of the adults of your generation, some of whom may still be alive.
All celebrating "together" because you love everyone, the more the merrier, is all very lovely as a theory but it doesn't hold water. You're drawing the line somewhere unless it's a closed loop because each set of older adults only has one adult child, who is married to an only child.
No matter where you draw the line, whether its at a gathering of six people or sixty, in many families this will be a cause of stress, offence, emotional blackmail and general dissatisfaction to someone.