A prayer of many rises before the Creator and the Creator crowns Himself with that prayer, since it rises in several ways. This is because one asks for Hassadim, the other for Gevurot, and a third for Rachamim. And it consists of several sides: the right side, the left, and the middle. This is so because Hassadim extend from the right side, Gevurot from the left side, and Rachamim from the middle side. And because it consists of several ways and sides, it becomes a crown over the head of the Righteous One That Lives Forever, Yesod, which imparts all the salvations to the Nukva, and from her to the whole public.
But a prayer of one does not comprise all the sides; it is only on one way. Either one asks for Hassadim or Gevurot or Rachamim. Hence, a prayer of one is not erected to be received like the prayer of many, as it does not include all three lines like the prayer of many.
Zohar for All, VaYishlach [Jacob Sent], Item 45
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A prayer of many does not depend on the number of people, but on the content of the prayer. Billions of people may cry out but if the cry is an egoistic one, the prayer is not answered.
A prayer of many is a request to reconnect the numerous pieces of the broken soul. This is the only prayer that the Creator answers.