Enhance calls home to the licence server periodically and reports the number of active websites - staging websites, soft-deleted websites, service websites, hostname websites, etc do not count. Our billing system will then create your next invoice based on your usage, $0.15 USD per website with a $5 minimum. Your first month is always $5 as you have no usage to measure at this point.
One licence covers your entire cluster, no matter how many servers you have. You can move customers between servers at any time.