I use Microsoft Power Automate (formally Microsoft Flow) to manage this.
1. Create New Automated Cloud Flow or from Template
2. Choose Trigger > When a Survey Response is Submitted (Survey 123)
3. Choose Survey (make sure to be signed into/connected to your AGOL account)
4. Add New Step > Choose Action/Operation > Send an Email (I use Microsoft Office 365 Outlook - V2)
5. Select Email Action and Customize Fields/Email Body as desired (add Dynamic Content - Expressions). This is where you should be able to add # of survey submissions (using dynamic content) but you might need to add a new field in the survey to include this info? Not sure.
6. Save and Run Flow Checker/Test
7. If you go to your Survey Settings in AGOL, you will see your Webhook is connected/On
Note: Make sure your Trigger and Action is signed into/connected to your AGOL account by checking Data > Connections > More Commands > Switch Account OR select Menu next Trigger/Action