De acordo com o documento de licenciamento do VS 2012 (e Assinaturas do MSDN) que pode ser encontrado em link você tem permissão para instalar o Visual Studio em praticamente qualquer número de máquinas onde você é o único usuário do software. Do texto da licença:
Where the Software Can be Installed and Run The licensed user can install and use the software on any number of devices. The software can be installed and used on your devices at work, at home, at school, and even on devices at a customer’s office or on dedicated hardware hosted by a 3rd party. However, the software is not licensed for use on 3rd party shared servers (“cloud” environments) or in production environments.