Only the UC license needs to be applied to the router. The SRST license is documentation that you did purchase them. It is not currently enforced in the IOS but I believe that is being looked at in the future. When you configure the router it will open those licenses up. It use to be an eval that turned into RTU. Now I think it is just RTU when configured.
We have router with FL-CME-SRST-25 so I believe we can add 25 srst phones and I have added around 10phones. I need to purchase more 25 license for same router in this case how can I add extra license to the route?
SRST license is a Right To Use (RTU) license. It means its not enforced in IOS and doesnt need any activation. So it means if you need to add another phones you just go ahead and order the licenses, there is nothing you will need to do on your router.