Well - good luck with that goal.. it is set pretty high
It's all about how well the application is designed & applied. It's really a case by case topic, and you'll know when you need to add another server to your rack. With 1m visitors, I'd imagine you'd need a couple of servers for sure. Once you get to the point where you go beyond one server, you'll need to look into load balancing (either hardware or software, both have their pros/cons), other server software/OSs (lighttpd, nginx, etc) to fit your needs. And when in doubt, hire a server management company to help you out, especially with an aggressive goal with such rapid growth.