ShareGate Overcast Enterprise is priced per tenant. A tenant is created to manage your Azure assets. Creating your tenant is required in order to have an Azure or SharePoint Online subscription. Being charged per tenant means that you will be charged for each Tenant ID associated with an Azure domain within your enterprise.
If you are not sure what your Azure tenant ID is, use our What is my tenant ID? tool.
For more information on what makes up a tenant, check out the What is a Tenant? blog by Microsoft.