Worth quoting a CA dev over at the TW centre.

Quote Originally Posted by C.I.F
Hi there

I'm sorry if I seem confused but why is this rumour about limited installs continuing? It's entirely false.

We have explicitly stated on our own TotalWar.com website that there will be no limit to legitimate installations of "Empire: Total War". An authenticated purchase of the game is all that is required.

When you install the game on to your PC there will be a one time authentication on "Steam". After that you can play with Steam set to "offline", if you dont want to get updates or patches of "Empire: Total War" or the "Steam" application.

You will be able to purchase a disk version of the game from most good game retailers - which will require the same one time Steam authentication - if you dont want to download the game from Steam.

If you get rid of your old PC after installing the game you can re-install the game, along with Steam, on a new machine and you will only need to authenticate the game, on Steam, to start playing it again. You can install it on ten PC's if you like but you will only be able to play it on one machine at a time.

Multiplayer online gameplay will require you to go online using Steam as Steam is the matchmaking service and the means of communication Empire uses between players.

Hopefully this is clear enough?
And again

We will not be limiting the number of uses of the game.
We will not be limiting the number of times you install and uninstall it - as long as multiple users dont try to use the same copy of the game.