Note that 1024 "limit" is from operating systems, not Mosquitto.

mosquitto.conf

@@ -228,8 +228,10 @@
 # a per listener setting.
 # Default is -1, which means unlimited connections.
 # Note that other process limits mean that unlimited connections
-# are not really possible. Typically the default maximum number of
-# connections possible is around 1024.
+# are not really possible. Typically operating systems such as Linux set a
+# default maximum number of connections possible at around 1024. If you require
+# more connections than this, the operating system limit must be increased with
+# something like `ulimit`.
 #max_connections -1
 
 # Choose the protocol to use when listening.