Some people choose to inventory their whistles by weight. If you know the weight of the container you're using (or if that container is of a negligible weight [like a plastic bag]), you can approximate the number of whistles you have by how much the container weighs.
To set up a system like this, you'll want to take measurements of various whistle amounts you know in advance. 25, 50, 75, 100, 200, 500, etc etc etc. This will give you an approximate weight per amount.
So if you have a scale that looks like this:
If you then weigh your full inventory and come up with 800 grams, you likely have around 400 whistles.
NOTE: This is most-accurate when using only one whistle type per container. If you mix and match whistles of different weights, your calculations will start to drift. If the whistles are similar in mass (2g vs 1.9g), this may not matter so much, but the difference is large enough, it will skew your calculations.