firestore snapshotlistener costs

Your snapshot listener will fire immediately with the 20 documents that match your query, and subsequently when the document changes.

You will be charged for 21 document reads in this case: the 20 reads for the initial snapshots and then the 1 read for the modified doc.

You'll be charged for one read for the changed document.

Frank's answer is correct in that the first time you activate your listener, you'll automatically fetch down all 20 documents, but each subsequent document change will only result in one read.