Gumby,
You're right that in the OT it often takes only one individual to be heard, but not any individual: the king, priest, prophet, "man of (the) god(s)," or the "righteous one" is supposed to be heard and be able to pray on behalf of others (intercession). As a result collective requests are rather trusted to the "right" person than directly expressed by a lot of people. After all "praying in Jesus' name" may be construed as a vestige of this ancient perspective in Christianity.