[Christian Kruse]

Kinda solved my problem.

I inserted a Slider Control into a Null-Object in my main comp.
Made a new comp with 25 coins and arranged them. Afterwards I used this expression on the transparency:
n = thisComp.numLayers;
if (comp("mainComp").layer("theNullObjectsName").effect("theSliderControllersName")("theSlidersName").value > n - index) {
transform.opacity = 100;
} else {
transform.opacity = 0;
}

Now I can build up/down my stack with one slider.
Hopefully this will be useful to someone in the future.