Microsoft Forms Pro: 2.000 responses counted monthly or annual?

[Tom1987]

Our organisation is an Office 365 tenant. We're analysing different survey tools and Microsoft Forms Pro is one of them. 

Unfortunately, the purchase information on this page: https://docs.microsoft.com/en-us/forms-pro/purchase is not clear. 

What is the real cost for only 1 license? And if our organisation only purchases 1 license, are the 2.000 responses counted on monthly basis or annual (24.000)? In one month our organisation can have 5.000 responses but the next month only 300...

[#2]

The way I understand it is that the response capacity is counted annually which means you would get 24k responses if you reach the limit (24k) in your first month you will get out of responses.