The system does not permit you to add more appointments if you go over the monthly limit.
There are two possible options to stop this from happening:
1. Subscribe to a superior plan
2. Buy prepaid appointments which will go into a reserve bank for busier months
For example, if you have the professional plan that has 250 appointments per month and you hit your limit for the month of January, the next appointments you book will be taken from your reserve bank of prepaid appointments.
Here's a common scenario:
31 December 2014
You have used 235 appointments for December. So you're okay.
25 January 2015
You have used 248 appointments for January. You are soon going to go over your monthly limit!
25 January 2015
You buy 50 prepaid appointments.
31 January 2015
You have a total of 260 appointments in the month of January.
Your reserve bank has thus used 10 of your prepaid appointments.
You now have 40 prepaid appointments left in your reserve bank.
This means that you were able to accept an extra 10 appointments for only a few additional dollars.
And on top of that, your prepaid appointments never expire and cost less the more you buy.
You can see the status of your monthly limit and of your prepaid reserve bank by clicking on your name in the top right corner of your schedule.