Maximum concurrent connections to SQL Server Express 2017 installed on Windows Server 2016 Essentials
@@MAX_CONNECTIONS

The @@MAX_CONNECTIONS function will according to docs:


  Returns the maximum number of simultaneous user connections allowed on an instance of SQL Server. The number returned is not necessarily the number currently configured.


So the above will give us some value the SQL Server is or was limiting the number of connection to.

On most servers this will likely return the default value of 32767.

Another two ways to access the value of the setting are as follows:

SELECT value FROM sys.configurations
WHERE name = 'user connections'


Or

USE master;  
GO  
EXEC sp_configure 'show advanced option', '1';  
GO
RECONFIGURE
GO
EXEC sp_configure @configname = 'user connections';
GO
EXEC sp_configure 'show advanced option', '0';  
GO
RECONFIGURE
GO


Why would I want to change this option

As I would suggest is the case with most advanced options, if you're asking the question it's unlikely you need to. The use cases are likely far between, but in general this setting can be used to limit the impact users are having on a server in a very high level sense.