TL;DR - you should be able to ignore partition key ranges and the number of them you have and just let Change Feed Processor manage that for you.
The partition key ranges is an implementation detail we currently leak. The short answer is we add new partition key ranges when we want to restructure how your data is stored in the backend. This can happen for lots of reasons, like you add more data, you consume a lot of RUs for a subsection of that data, or we just want to shuffle things around. Theoretically, if you kept adding data, we'd eventually split the range in two.
We're working on some updates for the v3 SDKs that are currently in preview to abstract this a bit further, since even the answer I have given above is pretty hand wavey and we should have a more easily understood contract for public APIs.