Empty Rooms

My wife and I decided to build this year and we have been having lots of unsettled discuss on the number of rooms we should build. I am of the opinion four (4) rooms is just Ok. While she says she wants six (6) - seven (7) rooms. I have no issue with blocks for building it or space but what's the essense of building many rooms when the kids will be all gone in say 20-25years time. We will now be left with empty rooms. My grandfather's house has over 12 rooms. Now most of those rooms are used as packing store as the kids have settled into their different families. The place is literally a ghost zone except for 2-6 days of Christmas. She says she wants a large family (she is an only child so I get where that's coming from) but in time, won't more rooms mean more emptiness? I need your thoughts on this please so we can resolve our differences and meet the builders. Thanks family.