Confluence Cloud pricing is a little different than Confluence Server licensing.
Confluence Cloud along with all other Atlassian Cloud pricing is paid on a monthly basis as a subscription model. That means the pricing for your Confluence site is based on the highest number of users who access the system in that month.
Atlassian holds your credit card details on file, and will charge you each month to keep your Confluence site and support going. Atlassian provides you with a billing estimate for each month as well so you are not unaware of what your charge will be for the following month.
See more information about Atlassian Cloud Pricing in this detailed document.