This is a very simple (and tiny) mechanism that combines redstone and hopper physics to create an item counter - a machine that counts exactly how many items are in the chest (or any other container). It outputs short signal for every item it counts. (but not too short - it slows down the items movement inside the containers to get enough time for enough long output signals)
The output signal can be connected to any counting mechanism that will count the signals and save the number of the items. If you don't know any mechanism like that, use mine: it has a screen-display signals output for a numeric 7 segment screen display.

You even don't need to download this mechanism, you can see it all clearly in the image.

I made this mechanism for the score-counter at the end of my adventure map: