How many users can I RDP into a windows server without having to purchase RDS/Terminal Server licenses?

Microsoft limits it to 2. If you need to allow 2 connections from the same user you will need to change the configuration. 

You can configure the number of simultaneous remote connections that are allowed for a connection. Restricting the number of simultaneous remote connections can improve performance of the computer because fewer sessions are demanding system resources. 

NOTE: If the RD Session Host role service is not installed on the computer, a connection can only allow a maximum of two simultaneous remote connections to the computer. (if you need information about installing the RD Session Host Role - then check here http://technet.microsoft.com/en-us/library/cc742813.aspx) 

Use the following procedure to configure the number of simultaneous remote connections allowed for a connection. 

Membership in the local Administrators group, or equivalent, on the RD Session Host server that you plan to configure, is the minimum required to complete this procedure. Review details about using the appropriate accounts and group memberships athttp://go.microsoft.com/fwlink/?LinkId=83477. 
To configure the number of simultaneous remote connections allowed for a connection 

On the RD Session Host server, open Remote Desktop Session Host Configuration. To open Remote Desktop Session Host Configuration, click Start, point to Administrative Tools, point to Remote Desktop Services, and then click Remote Desktop Session Host Configuration. 

Under Connections, right-click the name of the connection, and then click Properties. 

In the Properties dialog box for the connection, on the Network Adapter tab, click Maximum connections, enter the number of simultaneous remote connections that you want to allow for the connection, and then click OK. 

If the Maximum connections option is selected and dimmed, the Limit number of connections Group Policy setting has been enabled and has been applied to the RD Session Host server. 

You can also set the maximum number of simultaneous connections allowed for an RD Session Host server by applying the Limit number of connections Group Policy setting. This Group Policy setting is located in Computer Configuration\\Policies\\Administrative Templates\\Windows Components\\Remote Desktop Services\\Remote Desktop Session Host\\Connections and can be configured by using either the Local Group Policy Editor or the Group Policy Management Console (GPMC). Note that the Group Policy setting will take precedence over the setting configured in Remote Desktop Session Host Configuration. 



External Links on topic : 
http://technet.microsoft.com/en-us/library/cc753380.aspx