sort by tag count

hi
when i see tag archive posts like: http://XXXXXX.XXX/?tag=ABCD
i want to sort posts with tag counts
ex:
MyPost_1 has 3 tag: a and b and c
MyPost_2 has 2 tag: a and b

when i see http://XXXXXX.XXX/?tag=a , i want to see MyPost_2 before MyPost_1

how?