So, we, as humans, are limited by the laws of science: the laws of physics, the laws of conservation of mass, etc. However, the Christian/Jewish/Muslim God is not human. He is separate from time and space. He is not limited by our laws. He IS His law. He has no need to conform to ours. So, this means that He can do what we can't, which includes hearing the prayers of billions of people, and answering them (no is a valid answer). However, in other religions, miracles that occur are demonic ones. "And then shall be revealed the lawless one, whom the Lord Jesus shall slay with the breath of his mouth, and bring to nothing by the manifestation of his coming; even he, whose coming is according to the working of Satan with all power and signs and lying wonders (2 Thessalonians 2:8,9)." The snares of Hell attempt to drag these people away from God through their own works. We must always be on the lookout for these counterfeit miracles.
P.S. If this offended anyone because I used the bible as a valid source, suck it up. I know the bible is a valid document. If you don't agree, that's fine. Just don't be a dick about it in your criticism/insults/death threats.