"Who's counting," often said as, "... but, who's counting," is an expression
that roughly translates into, "the difference is insignificant." It is often said after a mistake has been pointed out.
Most often, though, it is said sarcastically, or, as a witty interruption or aside, to show that you possess some knowledge that the speaker does not.
It works most literally when applied to a number related subject, such as the price of an item, the current time, or the date of an historical event. It can be used in any situation, however, such as who said a particular line in a movie, which country signed what anti-pollution resolution, or whatever.