This is the main reason that A LOT of people are upset with this game at the moment. Because of the extreme anti-piracy measures they're taking. With your serial number, you can't install it more than 3 times. If you try to, you'll have to contact EA to get them to increase the number of game activations you can perform. But they don't HAVE to.
This is what DRM is about. You pay for the game, but instead get a mere license to play it. If their validation servers go down, you won't be able to install it at all, unless you crack it.