initial_dbms_default_primaries_count is a configuration parameter in Neo4j version 5.x, and its purpose is to specify the initial number of primary replicas (also known as primary instances) when setting up a Neo4j cluster. 

 

For example, If you've set the initial_dbms_default_primaries_count to 3 in the neo4j.conf file (<installation-directory>/conf/neo4j.conf) before starting your Neo4j instance for the first time, and you attempt to create a new user database without specifying its topology, this new database will be distributed with three primary copies across the cluster instances once the cluster is up and running.

CREATE DATABASE test;

Setting initial.dbms.default_primaries_count determines the default allocation during the initial startup. If it's configured as 1 before the first startup, all new databases will have a single primary (assuming topology isn't specified). If you later change this configuration to 3 and restart the cluster, it won't have any effect. To adjust the default allocation, please follow the procedure outlined below. This process will configure new databases with 3 primaries and 0 secondary instances. You should execute this procedure against the system database.

:use system CALL dbms.setDefaultAllocationNumbers(3,0)

More information on initial dbms settings and how they work can be found in the following knowledge article

Reference :

• https://neo4j.com/docs/operations-manual/current/configuration/configuration-settings/#config_initial.dbms.default_primaries_count
• https://neo4j.com/docs/cypher-manual/current/administration/databases/#administration-databases-create-database-topology