Firestore Reads Billing Explanation

Question

How many reads occur/billed for if the query for a stream requests, for example, 100 documents but the cache only contains 80 of those documents requested by the query since 10 new documents were added to firestore that fall into the query and 10 documents no longer fall into the query due to a limit constraint set on the snapshot request.

Answer 1

When a query reaches the server, the cache is not used. A query for 100 documents will return 100 documents, and cost 100 document reads, no matter what the cache contains.

I suggest reading this for more information about how the cache works.