You can use the mailstats contrib to see numbers of emails that have been flagged as spam overall and per user the number left in the junkmail folder, plus a lot of other information about the activity of the email system. However I cannot see the point of knowing how many spam messages from each sender, as spammers will change the sender regularly to try to fool the anti-spam measures.
Details of mailstats here:
https://wiki.contribs.org/MailstatsThe development version is for the new qpsmtpd and not yet released, so the latest version for SME9 will be loaded by:
yum update --enablerepo=smecontribs smeserver-mailstats
You get one report emailed to admin every night, it runs just after midnight.