Salesforce does not have a way of notifying anyone when the number of cases in the current month reaches a predefined limit for an account.
- Create a target field on Account called Number of Cases This Month.
- Create a source number formula field on Case called Case Count.
- Create a rollup with a filter that has the CREATED DATE Equals THIS MONTH criterion and add it to the rollup.
- Create a workflow rule on the Account page that evaluates the rolled up value against the threshold to determine if the email should be sent out.