how does PCR know THAT AMT OF DNA IS LESS?I WANT IT URGENTLY.

PCR does not know that the amount of DNA is less. It is an artificial set up or technique in which we feed the piece of DNA which we want to amplify and produce large number of its copies. For example, to detect the pathogen early, which is not possible by other diagnostic tests like blood screening as they need large number of pathogens to give clear and correct results. In this case, a small sample from body is taken say blood, the pathogens and their DNA is separated and fed into PCR which multiplies it in huge amounts. Then, these amplified pathogenic DNAs are used for other screening tests to detect the disease.  

The complete process is given below:
 

 Polymerase chain reaction (PCR) is a technique in molecular biology to amplify a gene or a piece of DNA to obtain its several copies. It is extensively used in the process of gene manipulation. The process involves in-vitro synthesis of sequences using a primer, a template strand, and a thermostable DNA polymerase enzyme obtained from a bacterium, calledThermus aquaticus. The enzyme utilizes building blocks dNTPs (deoxynucleotides) to extend the primer. In the first step, the double stranded DNA molecules are heated to a high temperature so that the two strands separate into a single stranded DNA molecule. This process is called denaturation. Then, this ssDNA molecule is used as a template strand for the synthesis of a new strand by the DNA polymerase enzyme and this process is called annealing, which results in the duplication of the original DNA molecule. This process is repeated over several cycles to obtain multiple copies of the rDNA fragment.