All these requests come at the same time. Since the database is eventually consistent (meaning the written data is not immediately available for reading after writing to the database) it takes a while for your server to catch up and block that IP address. That's why you get all these messages all at once.
To prevent that go to Components, Admin Tools for Joomla!, Control Panel, Options, Email Sending. Set Blocked request email throttling to Yes. The following three options control the maximum number of emails you will receive for blocked requests within a period of time.
Moreover, you can disable email sending for Suspicious Core Parameter. Go to Components, Admin Tools for Joomla!, Web Application Firewall, Configure WAF, Logging & Reporting. Find the "Do not send email notifications for these reasons" option and click on the dropdown icon to its right. Click on "Suspicious Core Parameter". Click on Save & Close.