The number of unique openings of the campaign is displayed on the statistics page for a sent campaign. See example image below:
Note that the number of openings at the individual level registers an opening every time the email client is updated. Even if the email message is forwarded, the openings are registered for the original recipient. Another common scenario is that some type of spam filter / anti-malware protection opens the email multiple times to search for malicious attachments or links. This is how the protection works in Microsoft Office 365, for example. The person responsible for the receiving email server can answer more questions about what controls they have.
For these reasons, the number may be perceived as abnormally high. Therefore, we normally recommend showing "unique" clicks, as each recipient is only counted once.