You want to create the detailed report that shows what users receive the most emails detected by the Anti-Spam service.
Email Security.cloud
Spam volume by user report is not available in the Email Security.cloud.
The Email Summary Report has a "Spam volume by domain", so the number of spam received, but this is per domain, not per user.
More detailed information you get with Email Detailed Report, but then you have to use formulas and work on the downloaded data to display the desired volume per user values, as there is no 'built-in' option.
Related article: Email Security.cloud Report types and data constraints