max_wal_senders

For DBAs and operators

WAL senders have their own process slots and do not count against max_connections (since PostgreSQL 12). A value above 0 requires wal_level ≥ replica; 0 disables replication and base backups entirely.

Refused connection, as the client sees it:

FATAL: number of requested standby connections exceeds "max_wal_senders" (currently 10)

• pg_basebackup -X stream (the default) uses two senders: one for data, one for WAL.
• A standby that lost its connection may hold its old sender for up to wal_sender_timeout, so a reconnect can briefly need an extra slot.
• On a hot standby the value must be ≥ the primary's; otherwise replay pauses with WARNING: hot standby is not possible because of insufficient parameter settings until the standby is reconfigured and restarted.
• The total max_connections + autovacuum_worker_slots + max_worker_processes + max_wal_senders has a hard upper limit: it must be less than 262142, otherwise the server does not start ("too many server processes configured").

SELECT pid, application_name, client_addr, state, sync_state
  FROM pg_stat_replication;

How to change it

Change in postgresql.conf or ALTER SYSTEM, then restart the server.