You can look in the PSTFile tables to see what stuff is migrating/migrated
But as for how many people are accessing items that are newly migrated, it's an almost impossible task
What you would have to do is enable auditing for items beig viewed then create a query that links the audit, enterpriseVaultDirectory and the vault store database together
And this would only work if you import the items in to a specific PST filer and not merge it with their regular archive
I.e if you have \inbox in your PST file, EV imports it to \PST Imported\inbox
As opposed to just merging the PST's \inbox with the archives existing \inbox
So a query would take the user and saveset ID, cross reference it to the vault store database and get the folder ID and cross reference it against the ArchiveFolder table in the directory database and put in a where clause that would be WHERE FolderPath LIKE '%PST Import%'
This would require auditing being enabled and it might grow pretty big
https://www.linkedin.com/in/alex-allen-turl-07370146