Hopefully you aren't working to the 21st Edition. In any case, I won't reconcile any math. I'll just say you will need 18 specimens. 9 weld metal. At the varying positions around the pipe, 6, 12, and 3/9 o'clock.
And be cognizant of other variables. Hopefully you don't have extreme ranges of thickness and diameter or it can get very expensive. I played with some numbers once on a project and hypothetically considered compliance to the 21st Edition. The impact testing was going to cost around a 1/4 million dollars. These people are insane.