I can comment on this from the opposite end of the spectrum.
My house was built in 2003 and as luck would have it, the original owners made material selections that generally stand up to the trends of today, nothing is the house looks dated.
As a result instead of doing a renovation of the bathrooms, we are just having someone come in and re-grout everything and re-caulk as needed.
There is 1 or 2 tiles (maybe less than 1% overall) needing replacement per bathroom.
Underneath the stairs in the basement were several large piles of spare tiles and bags of all the original grouts used.
Unfortunately there are a few missing tiles, e.g. the 12x12 tile used in the master bath, one is cracked and as per spares, I only have the half size for the perimeter, 6x12 tile.
The important part I want to point out is that the tiles are *completely* unobtainable. Even though I have the original boxes and packing slips, Google returns zero results - they're just not made anymore and there is literally zero record of them or replacements after 17 yrs.
So definitely keep some spares around for the future.. with the way trends and products change so quickly it could be as short as 3-4 years and you would be without replacements.