Measuring the need of Capacity or scaling SQL server
As others have said, there is no magic formula that will tell you how your database server will respond to a sudden "increase the volume by double the current one" (volume of what exactly, by the way).

The two things that can help you to get a general idea (neither of which you have, apparently):


Historical (over many months) data that show you the correlation between your server load, the volume of data, and the application workload (concurrent connections, user sessions, registered users, daily orders, or whatever makes sense in you case). "[P]erformance data for last 15 days" is completely useless.
Performance testing environment that's configured meaningfully similar to your production. In this day and age spinning up a virtualized test environment on one of the cloud providers and running it for a week costs peanuts, compared to the price you will pay for a production outage caused by unexpected performance problems. Performance burn-down tests should be a part of your release process. 


If you start preparing both of these tomorrow, in half a year you will be able to answer your own question with some degree of confidence.