An MSDN subscriber is entitled to activate as many copies as needed for his/her own development purposes. Therefore, if a computer enthusiast somehow has 20 computers at home which he uses himself for software development (and aren't acting as part of a business, for example, a server farm), one subscription allows all 20 of those computers to be running their own separate copy of Windows, Office, and any other Microsoft product. After a few installations, the activation keys will stop allowing automatic product activation over the Internet, but after a telephone call to the Product Activation hotline to confirm that the installations are indeed legitimate and consistent with the license agreement, the activations are granted over the phone.