what after WAL LSN value of type pg_lsn reach its limit

I am trying to determine the optimal Standby for automated failover based on the highest value of pg_last_wal_receive_lsn() of all slaves. But what if max_wal_size is reached, does pg_lsn values go to 0? Please suggest if there's any better approach to determine node with the best RPO out of all secondaries.