If you run out of characters on the free plan or the Starter tier you need to wait for the billing cycle to end in order for the characters to be reset or upgrade to a higher plan.
On plans higher than Starter (Creator, Independent Publisher, and Growing Business) you may enable usage-based pricing, which will provide access to additional characters.
The additional characters are billed at the end of the billing cycle, and when the outstanding balance exceeds 2 times the subscription price. If you go over this limit and decide to upgrade or downgrade your subscription, you will be immediately charged for this outstanding “usage-based” balance, as well as for the price of your new chosen subscription plan.