I think the answer to your question lies within one of the central beliefs of Christianity, as spelt out in the Apostle's and Nicean Creed ("We believe in the communion of saints"). You can find out more about that in Wikipedia, but in brief, what that means is that all Christians (alive and resting in peace with God) form one mystical body, with Christ as its head. In fact, I believe that the Roman Catholic Church, the Eastern Orthodox Church, the Oriental Orthodox Church as well as the Anglican Church and the Assyrian Church of the East point to this doctrine in support of their practice of asking the intercession of the saints in heaven (as well as a prayer request from a fellow Christian), whose prayers (cf. Revelation 5:8) are seen as helping the other members of the body.
Aside from the theological aspect of it, I think prayer requests form one of the cornerstone of building a faith community. Surely, God hears a lone prayer out there. But, as one body united, when one member suffers all suffers with him/her, as St. Paul wrote in his letter to the Corinthians (1 Cor 12:26). Hence, the need for fellow Christians to pray for one another.
God works in mysterious ways, and He always answer our prayers, though many times, not the way we were hoping for. Therefore, in my own humble opinion, I don't think the chances of a prayer being heard or answered in the way the petitioner wish for is determined by the quantity of people praying for him. It might be, but I can claim to know how God works... :)