Christophe Pettus explains max_wal_senders, a PostgreSQL parameter that limits WAL sender processes for replication, backups, and logical subscriptions. Since PostgreSQL 12, WAL senders have their own connection pool separate from max_connections, preventing application connections from blocking replication. The parameter defaults to 10 and requires careful sizing to accommodate pg_basebackup, logical subscriptions, pg_receivewal, and disconnected clients.