In Netbackup copy numbers are sequential, each new copy get is own copy number that is retained during is's lifetime. Expiring a copy does not "re-arrange" the copy numbers. Restore is always carried out from copy number 1 also know as the Primary copy. You can change the primary copy with the bpimage command.

As you have observed changes in the copy schema will result in different copy numbers. Using Netbackup Storage Lifecycle Polices is designed to handle multiple duplication and will secure each duplication is completed before the next start.

Hope this answers most of the questions.

You can always refer to the output of bpimagelist -L <backupID>.

Nicolai has explained how the number goes. 

Additional note, if you expire a primary copy (let say copy1), it's going to make the next copy to be primary automatically.

And, if you backup to disk as copy #1, then duplicate to create copy #2, then expire copy #1 =

copy 2 is automatically moved to primary. no copy 1.

If you then copy #2 you get copy #1, and unless you specify, #2 is primary until it expires, then copy #1 will be!

So - the copy numbers increment - but will fill in lower numbers if they are expired.

I believe the copy number is assigned at the start of your copy.

If you use SLP it is easier to control the timing of the duplications.

You will never have 2 unexpired copies with the same copy number.