1. A method for integrating multiple camera images to track a vehicle, adapted to a server having a request processing unit, an image capturing unit, an image processing unit, and an analyzing unit, for tracking the vehicle by using images captured by a plurality of cameras, the method comprising: receiving a request to track the vehicle by the request processing unit, wherein the request comprises a registration information and a position information of the vehicle; retrieving a plurality of images captured by the plurality of cameras in a specific range around a location of the position information by the image capturing unit; analyzing the retrieved images according to the registration information by the analyzing unit, so as to recognize the images comprising the vehicle, comprising: performing an image processing on the images captured by each of the cameras by the image processing unit, so as to recognize at least one license plate from the images; comparing the recognized at least one license plate with the license plate corresponding to the registration information by the image processing unit, so as to find out images having the license plate matched to serve as the images comprising the vehicle; estimating a recognition frequency threshold value indicating a minimum quantity of cameras that recognize the license plate by the analyzing unit according to at least one historical statistic value, wherein each historical statistic value comprises a license plate recognition accuracy of recognizing the license plate by n cameras, wherein n is a positive integer; calculating a quantity of cameras which capture the images comprising the vehicle by the analyzing unit; and issuing a message by the request processing unit when the calculated quantity of cameras is equal to or greater than the recognition frequency threshold value, wherein an expected value of expecting a same license plate to be recognized in the images captured by the cameras is set by the analyzing unit, and the recognition frequency threshold value is estimated according to the set expected value and the at least one historical statistic value by the analyzing unit.