It depends to some extent on what you're doing. If you're doing hunting practice, one at a time, and then go pull it, walk someplace different - even if only a few feet different and shoot again from a different spot.
If you're doing blank bale work for form, shoot as many at a time as you want, but remember that on a blank bale, you have to pick a different spot for each arrow, shoot your round, and then go pull them. If you insist on shooting at the same spot, you're not going to be concentrating on your form, you're going to be trying to hit that expletive deleted spot. The bale is an arrow catcher, not a target. The bale exists because you can't repeatedly dry fire a bow without hurting it.