Multiple standby nodes can connect to a single master. Set max_wal_senders to the number of standby nodes, plus at least one. If you are planning to use pg_basebackup -wal-method=stream, then allow for an additional connection per concurrent backup you plan for. You may wish to set up an individual user for each standby node, though it may be sufficient just to set the application_name parameter in primary_conninfo.
The architecture for streaming replication is this: on the master, one WALSender process is created for each standby that connects for streaming replication. On the standby node, a WALReceiver process is created to work cooperatively with the master. Data transfer has been designed and measured to be very efficient-data is typically sent in 8,192-byte chunks, without additional buffering at the network layer.
Both WALSender and WALReceiver will work continuously on any outstanding data to be replicated until the queue is empty. If there is a quiet period, then WALReceiver will sleep for a while.
The standby connects to the master using native PostgreSQL libpq connections. This means that all forms of authentication and security work for replication just as they do for normal connections. Note that, for replication sessions, the standby is the client and the master is the server, if any parameters need to be configured. Using standard PostgreSQL libpq connections also means that normal network port numbers are used, so no additional firewall rules are required. You should also note that if the connections use SSL, then encryption costs will slightly increase the replication delay and the CPU resources required.