Hi Snowbro,
You should be able to change the “Public Connections” parameter of the Create Session Blueprint node. This value in the online session is independent of the engine’s MaxPlayers config value for the GameSession, in most cases both values should be set to the same thing.
Note that currently the Null online subsystem doesn’t enforce the “Public Connections” value, but the MaxPlayers setting will still be enforced at the engine level. The Steam online subsystem should enforce Public Connections, though.