When it is difficult to run a cable to transmit video signals of the camera to the monitor, you might want to consider wireless transmission. But, the wireless signals may be attenuated or blocked by heavy metal walls, high voltage power lines, microwave equipment and others.
Most commercialized wireless systems have 4 or 10 channels; you can not use more than 4 or 10 transmitters in one area due to conflicts of the overlapped channel(s). But you can have as many additional wireless receivers as you want as far as they are within the range.