Not a direct answer to your question, but this thread is somewhat related and might be helpful: Depths for DR1 and more - Support / Data Products - Rubin Observatory LSST Community forum.
Also, this ancient (2013!) document refers to DR1 being expected to contain “23.09 billion objects”, but it doesn’t specify where this number comes from (it is appropriately less than the 37B predicted for DR11): DMTN-048: Qserv design prototyping experiments.