If you are going to go over your Keyword Allotment for the month, the system will warn you upon adding the additional keywords. You will have to agree to adding the keywords, as well as the charged overages.
Please refer to your contract for your overage rate. You can always choose to upgrade to a higher tier with a better keyword rate compared to the overage rate.
Emails will also be sent to the Administrators of the account, notifying them that overages have been incurred.