When you upgrade from the Free plan to the Business plan and attach your credit card details, you get a 30-day trial. You won't need to pay anything during that period. The subscription cycle for the account is set to yearly by default. You can change it to monthly while adding your payment method.
You will be charged automatically at the moment when the trial ends. You will pay for the number of seats present in the team at that moment.
InfoYou pay for the number of seats present in the team regardless of whether a user is assigned to it.