User Connections parameter is used to set the maximum number of simultaneous connections to SQL Server. However, the actual number of connections may be less, depending on database environment. The maximum value to which this parameter can be set is 32,767. However, this value is based on available memory and application requirements. Increasing user connections will increase the amount of memory needed for SQL Server overhead, thereby reducing the memory available for the data buffer and procedure caches. This is because each user connection requires around 18K of memory.
Also this value, along with “max worker threads,” is used to determine if SQL Server uses thread pooling. As a result, more physical memory may be required in order to maintain cache performance levels. Too low a value for User Connections may result in the SQL server servicing only a few clients at a time. Therefore, you should choose this value carefully after all of the above considerations.