Not Opened Email Min. Number of Times

Marketo logs the open email activity only 1x for the person-email combo i.e., if a person opens the same email more than 1x, Marketo will only log the Open activity 1x. Not opened email filter can check if the open activity is missing over a period of time, as a result for - 

• Min. Number of Times  = 1, would return people who wouldn't have any open email activities on the selected email would qualify, 
• Min. Number of Times  = 2, would return people who would have one or less than one email open activities on the selected email would qualify, 
• Min. Number of Times  = 3, would return people who would have two or less email open activities on the selected email would qualify,  and so on! 

It can be inferred that since opened activity is only logged 1x for the person-email combo hence SL would return same people for the Min. Number of Times constraint set to >= 2 when a single email is selected in the SL filter.

But when you set the Email constraint in the SL filter to "is any" / such that more than one emails qualify then,

• Min. Number of Times  = 1, would return people who wouldn't have any email open activities on selected emails
• Min. Number of Times  = 2, would return people who would have one or less email open activities on selected emails
• Min. Number of Times  = 3, would return people who would have two or less email open activities on selected emails, and so on.

Unlike the previous case, # of people may not be same for Min. Number of Times set to >= 2 as the SL filter is looking at more than one email asset.