My input image consists of 100 mens standing straight. I have slightly aerial view of the image. Whats the method to count the number of heads?
You know from your prior postings that it's always best to upload an image when asking advice on image processing. It only makes sense.
Lacking that about the only thing I can guess is to just do your basic image processing routine, which is
a process which I know you've done before. If the heads merge together you can try this algorithm by Steve to split some heads: http://blogs.mathworks.com/steve/2006/06/02/cell-segmentation/
I'd first try to find the faces by finding color that is face color. Then filter based on shape - get rid of long skinny things (the arms). It won't be perfect but you should be able to get most of them. See my image segmentation demo and my color segmentation demos if you need to know how to find colors or filter out objects based on their properties.