Hello,

if the object is placed in midway of the mirrors then the formula
n=[ (360/A)-1 ]
n-number of images seen, a- angle between the mirrors, will give you the number of images actually seen in experiment.

but if the images seen are odd in number, the image at the center is actually the combination of 2 partial images each on each mirrors. hence if you want the partial images to be counted separately n= (360/a).
in case the images seen are even in number then all images are seperateand there are no partial images hence n=[ (360/a)-1 ]

For more clarity please watch the video in the link below. 

https://www.youtube.com/watch?v=6maFbmPCnzY

Hope this helps.....