For example, if I sent out 8 emails in a campaign, can I find out the average number of emails the unique subscribers opened?
Yes. Run a lead performance report grouped by email address - filter to leads sent the emails you want to report on. Use a smart list custom column for each email open.... You will see which emails each individual opened. Export to Excel and get your average opens per lead.
Not really?
If you do an Email Perf report, for 8 emails, you can see an avg open rate or count. Not unique.
In RCE, I believe it is unique.
Smart List will give you a list of people who Opened at least one of the emails.