It is impossible to tell exactly how many subscribers successfully receive your email campaign in their inbox, because not all ISPs and corporate mail administrators send non-delivery bounce notices and some subscribers may have set up their own spam filters, which can prevent your email from reaching their inbox.
We do report the exact number of email addresses that the email campaign was sent to, and we report the exact number of email addresses where we receive a bounce notice. We cannot report a truly 100% accurate number of "Opens" due to issues with text emails and ISPs and email clients that strip away the open tracking feature.
So the most important thing you can do is look at your open rates over time - are they improving, staying still or getting worse? Even if the absolute numbers are not 100% accurate, you can assume the measurement is consistently out between campaigns. So the trend is more important than the individual figures.