A nickel has a diameter of 21.21 mm and a thickness of 1.95 mm
2.121 cm diameter
0.195 cm thickness
Think of a nickel as a cylinder
r = 2.121/2 = 1.0605 cm
V = pi * r^2 * h
V = pi * 1.0605^2 * 0.195 cubic cm
2 liters = 2000 cubic cm
2000 / (1.0605^2 * 0.195 * pi) =>
2902.84713216
Your upper limit is 2902, but it could be as little as 0 (for instance if the 2L bottle was only 1 cm wide, then you wouldn't be able to fit a single nickel in there)
I'd say that you could expect about a 70% packing efficiency, just off the top of my head
2900 * 0.7 =>
290 * 7 =>
300 * 7 - 10 * 7 =>
2100 - 70 =>
2030
I'd expect around 2000 nickels would fill up the bottle quite nicely, supposing you can get the nickels into it.