One cleanse clears what's there today. It doesn't stop your dog from picking something up again next month.
And dogs do pick things up again. On walks, at the park, from the yard, from other dogs. Most of the time there's nothing to see at first, no scooting and no licking, so you don't find out until it has already settled in.
Six bottles keep your dog on a steady cycle, so the drops are already on dinner when that happens instead of sitting in a cart you meant to come back to. You stay ahead of it. You're not starting over every time the scratching comes back.
Is it more than you need today? Maybe. It's also the lowest price per bottle, it ships free, and the 90 day guarantee covers your first order either way.