It depends on how densely they're packed. But to get a first-hand estimate, do the following:
1- Determine the volume of a quarter. See information from the US mint.
Diameter: 17.91 mm Thickness: 1.35 mm
To get the Volume, use this formula:
(pi) r²h
V = (pi) r²h
V = 3.14 * (8.955)² * 1.35
V = 339.93 mm³
2- Get the volume of 750ml in cubic millimeters:
750ml = 750,000mm³
3- A limit of how many coins would fit is: 750,000mm³ / 339.93 mm³ = 2,206.
But because the coins wouldn't pack that densely, that many would not fit.
4 - Packing circles in a plane would give a density of about 0.9. If you pack the coins neatly into the drum, you'll probably come close to this.
So, there would fit (2,206 * 0.9) about 1985 coins.
Using math and science to inform your estimate could win you some tasty treats, although there may be errors along the way. Here are a couple of easy tricks to send you home with candy you desired so badly that will resort to the use of math in public.