We had huge servers, with the database and the application itself running on separate instances. IIRC, we had a 32 core, 64GB instance just for the DB itself which we doubled when we started adding more sites to our configuration and it still wasn’t enough. As for the numbers, our site(s) get heavy traffic everyday, in millions daily, since we are a stock broker.

You’re right about MySQL performing alright for 10-20m rows, but from our perspective those numbers are not that big for a company this size.