Wafers come in batch sizes of a FOUP, these are the little sealed boxes you see the robots on the tracks moving around a semiconductor factory. One might only need one wafer of chips or less for a project, those other unused wafers go in storage after some testing. When the military needs spare parts, those wafers can be brought out of storage and diced and packaged (if we will even still do that in the future).

This technique is already done for things with long operational lifetimes. It would be nice if we made a distributed (geographically) wafer bank so that we will have a long supply of semiconductors, esp after a civilization scale catastrophe.

https://en.wikipedia.org/wiki/FOUP