Are you asking about how many alerts were sent to each sales rep in a defined period? If so, there isn't an off-the shelf report that will show this and there isn't a "by lead owner" option on any reports. What I have done to generate a report of leads that have been sent a certain email by salespersons or in your case, leads that generated an alert email to the owner, is as follows.
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Create a Lead Performance report with the desired smart list filters.
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Create a set of smart lists for each salesperson with filters for the desired action (e.g. alert was sent, sales email was sent).
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Add these smart lists as custom columns to the Lead Performance report.
The total at the bottom of each custom column will give you a count of the number of leads that generated the activity by owner. Keep in mind that owner changes during the measurement period may result in multiple attributions to different owners and that multiple activities during the measurement period will list only a single lead.