What determines the maximum number of concurrent connection to SQL Server 2016 Express
So are so many factors because there are so many different types of loads.  When it get loaded the only defense mechanism is to deny connections.  

I you are looking for max connection you are looking for the wrong thing.   

Design your application to be scalable - acquire the connection as late as possible and release the connection as soon as possible.  A well designed application could have 100 active users and never have more than 10 active connections.