If you mean the app store version, that's different. You can install it on as many macs as you own. However, it's not a good idea to buy the app store version.
It doesn't have the organizer, which may be a plus or a minus, depending on your point of view, but more importantly you can never update adobe camera raw (well, you can, but it's a highly complex process involving hacking the program files), you can't install any third party products that require deleting mediaDatabase.db3, and once a new version of PSE/app store comes out, there's no way to download your version again. (For other versions there are links on adobe's support site, but they require a serial number, which you don't have with an app store purchase).
