Yes, reads are not released (i.e. are blocked) until sealing is complete. We call the minimal set of nodes sufficient to serve reads for a log (the same set is needed for sealing to complete) an f-majority.
For a simple case where placement of data is location-agnostic, indeed the definition of f-majority is n - r + 1, where n is the nodeset size, and r is the replication factor.
However, if your replication property, is say, "place 3 copies across 3 racks", then the definition of f-majority becomes more complicated - e.g. having all nodes in the nodeset respond minus two racks will also satisfy it.
For a simple case where placement of data is location-agnostic, indeed the definition of f-majority is n - r + 1, where n is the nodeset size, and r is the replication factor.
However, if your replication property, is say, "place 3 copies across 3 racks", then the definition of f-majority becomes more complicated - e.g. having all nodes in the nodeset respond minus two racks will also satisfy it.