If an object is placed between two parallel mirrors, then the number of images formed will be:
Infinite
When the incident ray from an object falls on the mirror, the reflected light travels in the opposite distance from that mirror. It then hits the second mirror and reflects back in the same direction. This happens for infinite number of times and hence, we see infinite images.
In other words, the image formed by the first mirror acts as an object for the second mirror and this continues infinitely.