How to set up alerts once the sheet has reached a certain row

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Hello - I understand there's no capability to limit the number of form responses - only workaround. Can someone please help me set up alerts once a sheet has reached a certain row number? I am setting up a Smartsheet for an event and only have 30 spots available. How do I set up an alert when the form responses reach 30 so I can go in and deactivate the form?

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