• http://technet.microsoft.com/en-us/library/ms143432.aspx

    User connections | 32,767

    http://technet.microsoft.com/en-us/library/ms187030.aspx

    SQL Server allows a maximum of 32,767 user connections. Because user connections is a dynamic (self-configuring) option, SQL Server adjusts the maximum number of user connections automatically as needed, up to the maximum value allowable. For example, if only 10 users are logged in, 10 user connection objects are allocated. In most cases, you do not have to change the value for this option. The default is 0, which means that unlimited user connections are allowed.

    I assume, 0 means unlimited connections up to the maximum value allowable.