Short answer is add more hardware.

You haven't given anywhere near enough information to take a guess, and this isn't something that will be answered in a forum.

Are there bottlenecks? How does the hardware perform? The application? 20,000 users an hour doesn't mean anything. Do they read rows? Do they all insert 1 row? 10 rows? multiple tables? Identities go up to 2B rows, so 10mm isn't a lot.

If you are worried about scale, engage a consultant. I can recommend someone if I know where you are.