Multiple instance on single SQL Server We have a server running single instance of SQL SERVER in production environment. 

We are planning to create another Instance of SQL SERVER in same server.

My question do we have to buy a new license or the existing license is good enough ?
You can install as many instances of SQL Server as you like on a server, up to 50. The licensing is by the server, not by the instance (it used to be, once upon a time) so you have some freedom to do so.

The real question I would counter with is: Do you need another instance? Most people don't.