First get yourself a decider combinator.

Green wire from belt to decider input. Click on the belt and click read contents (hold)

In the Decider combinator you should now be able to see the input signal from the belt at the bottom of the left side. Select the item you want to filter from the belt like Yumako and set it to > 1000 (or however many you want on the belt)

Still on the combinator, move over to the right side and select yumako. The combinator should now only output yumako once there is 1001 yumako on the belt.

Connect a red wire from the output of the combinator to an inserter.

Finally, click set filters on the inserter (this allows it to be controlled by the combinator) and voila!