How to count unique accounts from the campaign member related list?

There is no configuration through which this can be achieved easily. This can be done via a trigger.

Create a trigger on CampaignMember for after insert, after update, after delete and after undelete.
Get the unique campaign Ids from Campaign Members
Create a aggregate query and group by Campaign Id and Account Id.
The aggregate query should be able to give you the unique count which you can update on Account. You might also require triggers on Contact and Account for handling situations such as Account update on Contact, Contact deletion, Account deletion.