By default only two concurrent active rdp sessions are allowed on a windows server 2012.  If we need more than two  users simultaneously log into the server  we need to have Remote Desktop Licensing (RD Licensing), formerly Terminal Services Licensing (TS Licensing), which  manages the Remote Desktop Services client access licenses (RDS CALs)  for users to connect to a Remote Desktop Session Host (RD Session Host) server.

In short if  more than two rdp connections then this is no longer remote administration, you are running a remote desktop server and providing applications to users, which means you need RDP Client Access Licences for each user.

Before proceeding with the install first we have to purchase the Microsoft Windows client access Licence ( Remote Desktop service- User CAL) from Microsoft store. After purchase we will get a product licence key and this Key is used during installation of services in the server.

Now if you are trying to implement  RDP Client Access Licences  on a VPS/Dedicated  environment that is purchased from a Hosting Provider  instead of purchasing  RDS CALs directly from Microsoft website you have to use SPLA SALs, for which you pay a monthly fee to the Hosting Provider.

Once we either have the Retail Purchase Product Key or SPLA Agreement Number follow below steps.

Open "Server Manager" >> Click "Add Roles and Features"

Click Next
Select "Add Role based or Feature based Installation"

 Select "Select a server from the server pool". Click Next

 Scroll down and select Remote Desktop Services then click Next

 Click Next without selecting a Feature
 Click Next
 Select the "Remote Desktop Licensing" and "Remote Desktop Session Host"

 Click Add Features on the pop up
 Click Next
 Select the Restart the destination server automatically if required then click Install
 Once finished click Close.

Normally up to this point we concluded Installing the Remote Desktop Licensing Role

Now lets Proceed with Activate Licensing Server

launch the Remote Desktop Licensing Manager.

For that Go to Start -> Control Panel -> Administrative Tools -> Remote Desktop Services->
 Remote Desktop Licensing Manager
Right click your server name then click Activate Server

Click Next

Leave it on Automatic connection and click Next

Enter your information and click Next

[Optional] Enter additional information and click Next

 Check the box to Start Install Licenses Wizard now then click Next
Accidentally If we didn't cliked the "start Install Licenses Wizard" in the previous step it can be access 
by right clicking your server name in the RD Licensing Manager then click Install Licenses

Click Next

Pick the type of license you want to install.

Choose Licence program as "Licence Pack (Retail Purchase)", if we purchased RDS Licence directly
 from Microsoft

Enter your license key then click Add. It should populate the License Codes Entered section, 
if the license key was correct. Click Next
Click Finish to complete the license install.

One point about licence key is, suppose if we purchased the licence key for windows server 2016, 
the same key can be used for all older windows server editions like 2012, 2008 editions. 
Older version product key will not work on new editions.

Choose Licence program as ” Service Provider License Agreement” if you are in a VPS/Dedicated servers
provided by a Hosting Provider and they already have a data-center Licence and our VPS Operating System
licence comes under it.

Enter your "Agreement Number " and Click Next

Select "produt version" as per your Windows Operating System Edition
 Select Licence Type as " RDS Per User CAL" or "RDS Per Device CAL" According to the Licence 
type given by your Hosting Provider
 Choose the Quantiry as per your requirement. Say if you need to have 5 users simultaniously log into 
the server. Type 5 and Click Next
 Click Finish to complete the license install.

The New license key we just added will show in the "Licence manager" itself.

Go to Start -> Control Panel -> Administrative Tools -> Remote Desktop Services -> RD Licensing Diagnoser.
 This will show if there any issues with Installed Licence
Now Open RUN, and enter gpedit.msc

Go to section
 Local computer Policy>> Computer Configuration\Administrative Templates\Windows Components\Remote Desktop
 Services\Remote Desktop Session Host\Licensing
Double click on "Use the specified RD license servers" >> click Enable, then Enter server name
 and click on Apply

Get name of Server from RD Licensing Manager.

Now Double click on "Set the Remote Desktop licensing mode" >> click enable, then select "Per User" or
 "Per Device" as per your Licence and Apply

Now restart Remote Desktop Licensing service from the Windows services section. Normally this concludes the
 Installation, Configuration and Licence Activation for RDS CAL.