One software license is required per Web Front End Server per year.

Web Front End (WFE) servers are the servers in your deployment that run the Web Application service. You could determine the number of WFE servers from SharePoint Central Administration website or contact your SharePoint administrator. You must activate each WFE server in your deployment with a unique license key.

Licensed on an Annual or Perpetual License.
The Annual Subscription License requires you purchase an Annual License each year to continue using the software.
The Perpetual License allows you to continue using the software after the Subscription period has expired however you will need to renew the Subscription to continue to receive support and upgrades.

Subscription includes all updates and support during the subscription period.