At the cost of 1 Action given by its Handler,a Dog Companion can execute one of these
Instructions...
Most people seem to think the Dog Companions are already overpowered, so letting all of them attack for a single action might be a bad idea, but it's your game, so if you want to play it that way, go for it.
Notice the bolded word...doesn't say a single action. Every wording to me says 1 action for 1 companionI was reading there and it says: "A Handler can spend his own Actions to give Instructions to his Dog Companion(s).". Seams to me that an action can affect more than one dog companion.