The command will be something close to
logparser "select TimeGenerated, count(*) from security group by TimeGenerated"
But you will need to use the substring function on TimeGenerated to chop off the time and leave just the date. You can download logparser at http://www.microsoft.com/downloads/details.aspx?FamilyID=890cd06b-abf8-4c25-91b2-f8d975cf8c07&displaylang=en and there are examples of its use all over the Internet.