New features of PG 12 Connection slots and WAL senders, detailed below:

Move max_wal_senders out of max_connections for connection slot handling.Since its introduction, max_wal_senders is counted as part of max_connections when it comes to define how many connection slots can be used for replication connections with a WAL sender context. This can lead to confusion for some users, as it could be possible to block a base backup or replication from happening because other backend sessions are already taken for other purposes by an application, and superuser-only connection slots are not a correct solution to handle that case.This commit makes max_wal_senders independent of max_connections for its handling of PGPROC entries in ProcGlobal, meaning that connection slots for WAL senders are handled using their own free queue, like autovacuum workers and bgworkers.

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In PostgreSQL, if the client connections exceeds a size max_connections parameter setting, the client connection will be rejected.

psql: FATAL:  sorry, too many clients already

To deal with this, version 11 or earlier PG provides superuser_reserved_connections parameters, provides a backdoor connection for the super user (administrator), but only for the super-user only, and does not provide backdoor to copy the user, if you exceed the maximum number of connections then copy to be wrong needs to be connected, resulting in the destruction of the HA environment. 
in order to ensure that the resources HA environment "HA", in PG 12, copy the connection needed to manage alone, no longer connected with the normal client (max_connections control) share the same Array (ProcGlobal). At the same time, this feature requires not less than the standby machine max_wal_senders max_wal_senders host or it may cause a malfunction.

References 
Postgres 12 highlight - Connection slots and WAL senders