the couples get rooms (usually with their baby) or sleep by someone else and the grandkids all sleep together in the basement or living room etc... boys room and girls room. and you take turns but try to get as many people in at once as possible. this was how I grew up. and then eventually the children start making their own yom tov and the grandparents take turns going to them.
Right now with my familt and my inlaws ints not such a problem. My inlaws have 3 bedrooms aside from their own and a basement and a den. And there are 3 married couples that come for yomtov. and one has inlaws in close distance (married to on only child). and there is only one baby. the next 2 sibs in law are not going to be on the market for a few years so we will see how it ends up then.
Im the oldest. only one married. my parents have 5 rooms that can be used as bedrooms (private- door locks etc) and a BIG room in the basement so that wont be a problem for a while. we have had 2 other familys plus grandparents with us befoer and we squish and share rooms for a week.