Potato, I'm afraid your opinion is biased.7, they would probably all survive, I mean the other 6 get only 1/6 less water. I don't know exactly how much water they need, we just water them.
Anyway, I would just get more water from the tap and if I can't afford water I wouldn't have bought 7 plants.
But answering your question with another method, put the 7 plants together if they are all same size, give all the water, let them do it. If some are much smaller then I would give them water based on how much I think they need and so water to all 7, or you could give it to the 6 that you want.