One. Similar to most software, each installed instance of the program requires its own license. In other words, if you want to install it on 2 different domains you must own two licenses. However you are allowed to install one additional copy for web development, testing or backup purposes and as long as it's not used publicly. Please note that if you would like to change the domain name, then you don't need to buy another license.
If you plan on running multiple sites, you may purchase an unlimited license addon which allows you to run unlimited number of sites.
