According to your last comment:
But my question is when i try to make a reference to a collection to get the size of that collection,how it effect to the firestore billing?
If you only create a reference to a collection, you won't be charged but if you create a query that returns 5 elements, you'll be charged with 5 read operations.
According to the official documentation regarding Cloud Firestore billing:
There is a minimum charge of one document read for each query that you perform, even if the query returns no results.
So you're also charged with one document read, even if your query does not return any results.