No, you do not have to purchase a Virtru license for each device you use. Each Virtru license is associated with a single email account, not to a specific device or computer.
If your email account has a Virtru subscription - or is a member of a customer's Virtru team - you can use a Virtru plugin, add-in, or app, on as many platforms as you would like (mobile, web, and desktop).
If you are interested in purchasing Virtru, please contact sales. To purchase additional licenses for an existing account, please reach out to your Virtru Customer Success Manager or contact support.