Synchronous replication offers the ability to confirm that all changes made by a transaction have been transferred to one synchronous standby server. This extends the standard level of durability offered by a transaction commit. This level of protection is referred to as 2-safe replication in computer science theory.
Does this mean that Synchronous replication only works with one standby?
Thanks in advance
No, it means that it requires, at minimum, one standby to confirm that the transaction has been committed there before the master confirms it back to the client. Look at this section of the docs