The mathematics of prayer is this: When 2 people pray, the effect on each person is as if 4 people were praying, 2 for one and 2 for the other.
If there are 100 people praying, the effect is as if 100 people PER PERSON was praying.
In Bettie Eadie's NDE, as she rose from the Earth, she described what she saw: beams of light. Asking her guide what they were, he replied: they are prayers. The narrow ones are from one person praying, the wider beams of light are from many people praying. The intense brilliant beams are from those in dire need.
I thought it was a neat idea to think about.
The negative thing that happens when you ask someone to pray for you is whatever their evil habits are that they are not cleansed from are put on you.
Someone here at this site asked if anyone needed prayer. Curious, I put in my name, and that night I had a dream about her religion. It was dark, and its leaders had turned their backs on them and could no longer help them. They were decrepit and old, and of the 5 or 7 old men standing there, only one was 1/2 turned toward the people gathered. It was really weird. I believe this was the effect of that person praying for me.