Well, actually it depends on how big the batch you make. The recipe is for a "batch" but you need to make enough to cover the bird and that depends on how big they are and what you put them in. It's been a long time since I totaled the volume of the brine and how much it takes, maybe someone has their notes with them. I'll calculate it out this year.
I have the pickle barrels for my birds and it's usually 2 or 3 batches to fully cover.
Spatchcocking will reduce the shape and allow you to brine more with less. But it's cheap enough to make.