Your credit card is billed monthly for the seats in your subscription.
Let's say you have 30 users, then you'll pay for 30 seats a month. The price drops per user and you can scale or downscale at any day. Billing is per month. When you pay for 100 seats and downgrade to 20, you'll receive a refund for 80 seats for the remainder of that month. When you upgrade you'll only pay for the extra seats that month. Contracts are flexible and cancellable every day.