Products registered SINCE March 25, 2014 support activations on up to 3 computers concurrently.
Products registered BEFORE March 25, 2014 are single-activation only. However, you can order a license with 3 additional activations for any of these titles for $29/29€.
You can purchase an Additional Activations Pack by clicking on the '+' sign on your My Products page.
To check the number of activations on your license and how many remain available, please refer to the article How many activations do I have?