Everyone makes good points, but IMHO, your practice should match how you play.
For hunting & 3D, the one arrow at a time practice is essential. In both of those you have to make the best shot you can on the first and only arrow.
For paper punching target shooting, it depends on which particular game you want to play. I shoot a lot of 5 spot, so my 5 arrows at a time work for me. If I was shooting a Vegas target, it would be 3 at a time or for FITA, it would be 6 at a time.
One reason to shoot the number that you compete with is that you have to condition yourself to shoot multiple arrows with good mental focus. When I first started 5 spot, I noticed that my first arrow would be gread, the next one good, the third one bad, the fourth one also bad because I was flustered from the 3rd and my 5th one would be good because I had gotten myself back under control and focused.
I've since come to understand that every arrow has to be shot with full focus and conditioned myself to do it.
This isn't better or worse that one arrow at a time shooting. It's just a different aspect of archery that requires different training.
We've all heard of guys that were deadly on fur, but horible when shooting at paper. In many cases it's not because they are poor archers, it's because they aren't mentally conditioned to shooting multiple arrows. It is a skill that hunters and 3D shooters don't need, but target archers do.
Practice how you play.
Allen