Go to the menu "Marketing" and select "Emails". Search for the name of your email campaign using the search field. Then press the gear button to the right of the desired campaign.
Press the button "Statistics."
This will take you to a page where you can see information related to your campaign such as: total emails sent, total opened, total opened, total not received, and # of times opened (categorized by type of member: active, lead, or dropped)