dynamic stamp with an incremental number

A global variable works, but I think a better option is to store the number value in "doc.info", so that last increment is persistent with the document. Meaning the stamp will always start off at the last increment, even when Acrobat is closed and reopened. It also makes it easy for you to set/change the increment by simply editing the values on the "Custom" tab of the Document Properties dialog.

The stamp script should be something like this:

event.value = ++event.source.source.info.myInc;

I just made a github of an auto-incrementing stamp set that should work:

GitHub - q335r49/AcrobatIncrementStamp: Auto-incrementing stamp for Acrobat

Take a look here for some more information about this subject: Auto Increment Exhibit Stamps - saving metadata to Stamp file (JavaScript)

You would need to set up a persistent global variable that keeps track of how many times the stamp was applied and increments its value after each time.