I have about 1500 clients in my Organization. I am planning to convert sever 2003 platform to server 2012. I have everything required for server 2012 platform. All of my clients license are original. Most of them are using Windows 7, 8 and 10. So I want to know about which version of server will be appropriate for my organization? And as my clients have original windows is it necessary to buy CAL for every single clients? Please help me.

Hi

 User CALs needs to be same or higher version to Server OS.(if server is 2012 r2,you need 2012 r2 User CALs).And you have to purchase each number of User CALs.

Also check these

https://www.microsoft.com/en-us/licensing/product-licensing/client-access-license.aspx

http://blogs.technet.com/b/volume-licensing/archive/2014/03/10/licensing-how-to-when-do-i-need-a-client-access-license-cal.aspx

You could use Windows Server 2012 r2 with your win7,8,10 clients without any issue.i recommend that you always prefer to use newest OS.