Answer
$1.3\;\rm L$
Work Step by Step
The piston is applying a volume stress on the water. And we know that the volume strain of water due to a pressure is given by
$$\dfrac{\Delta V}{V_i}=\dfrac{-P}{B}$$
So, the number of liters of water that come out to atmospheric pressure after the safety plug on the top flies off is given by $\Delta V$.
$\Delta V$ is the amount that comes out to neutralize the pressure between the vessel and the atmospheric pressure.
$$\Delta V=\dfrac{-P}{B}V_i$$
Plugging the given (see table 15.3 for Bulk modulus) of water;
$$\Delta V=\dfrac{-2000\times 10^3}{0.2\times 10^{10}}(1.3)=\bf -0.0013\;\rm m^3$$
$$\Delta V = -0.0013\times 1000=\color{red}{\bf -1.3}\;\rm L$$
