From the list of devices (at the top), select the mobile number the limit should be applied to.
From the left menu, click Limits & Controls to expand.
Click Limit text.
Click No limit to uncheck the box.
From the texts field, enter the message limit.
Once the limit is reached, text messaging is restricted (except to/from trusted contacts) until the next billing cycle.
Your child receives a free text alert when they're within 15 messages of the limit; you receive an alert once they've reached the limit.