max_replication_slots

Specifies the maximum number of replication slots (see streaming-replication-slots) that the server can support. The default is 10. This parameter can only be set at server start. Setting it to a lower value than the number of currently existing replication slots will prevent the server from starting. Also, wal_level must be set to replica or higher to allow replication slots to be used.

Note that this parameter also applies on the subscriber side, but with a different meaning.