I need to pull up how many users logged in to Sitecore CM per month (for the past 6 months). Is there a way I can do this through Sitecore or maybe in core db, which table should I look at?
If you need it per month then the only way I know is to check the Sitecore logs and search for
AUDIT (<username>): Login messages. This is only available if you have the logging at
aspnet_Membership table only the
LastLoginDate is tracked by Sitecore.
If you only have the database as a source you can get some rough numbers through SQL query. For example the following query shows you, how many updates happened per month per one user.
SELECT Updated_Month = DATEADD(MONTH, DATEDIFF(MONTH, 0, Updated), 0), Value, COUNT(Value) FROM [master_db].[dbo].[VersionedFields] WHERE FieldId = 'BADD9CF9-53E0-4D0C-BCC0-2D784C282F6A' /* __Updated by field from Sitecore */ GROUP BY DATEADD(MONTH, DATEDIFF(MONTH, 0, Updated), 0), Value ORDER BY Updated_Month
you could use this module https://marketplace.sitecore.net/en/Modules/S/Sitecore_History_Viewer.aspx to get the list of users that have made some change in CMS.
You can also modify the code to suit your requirement. This may be a very old module but it works for Sitecore 9 as well