You will see one activity of each type per SFDC Contact/Lead per day. It’s logged as an SFDC Activity.
Because only one type-activity-person combo is synced per day, there’s no guarantee that a specific email sent on Day N will be the one synced to SFDC on that same day. Of course, the fewer emails you send to people, the more likely it is that the one you’re interested in will be the one that makes it over to SFDC.
In general, Marketo Sales Insight is a far better choice if you want to see Marketo activity data within the Salesforce UI. With MSI, a person’s full Marketo Activity Log can be seen by sales, though it’s not actually stored in SFDC.