You need 1 product license for each SharePoint Server.

SharePoint executes workflow instances in one of two places depending on the last action. If the last action in the workflow was waiting on a user input, the workflow continues to execute on the SharePoint Front-End Web Server where the user completed that input. If the workflow is continued from a delay timer or from an event being received elsewhere, it executes within the SPTimer service on any SharePoint Server where SPTimer service is running. 

In SharePoint 2010, you can set the preferred server where the SPTimer service runs. In this case, you need 1 product license for each SharePoint Front-End Web Server and 1 product license for each SharePoint Server where SPTimer service is working. To do this, click in the Manage Content Databases menu of the Application Management section of SharePoint Central Administration. Then click on your content database and scroll down to the setting for Preferred Server for Timer Jobs. You can also manually stop the SPTimer service on any servers you don’t want it to run on.