When you've used more than 90% of your monthly appointment limit, you'll see a note after you create each appointment alerting you about how many appointments you have left. All reminders will be sent successfully for any appointments you've created, and you'll be able to upgrade if you want to create more appointments for the same month after reaching your limit. Once you hit your limit, you will not be able to schedule additional appointments until the next month or until you upgrade. This way you will never be charged overages.
Note: The number of appointments during the calendar month (from the 1st until the last day) is counted towards the monthly appointment limit. This means that the limit will reset at the beginning of each calendar month, regardless of when your billing cycle starts and ends.