Voltage gained by expelling electrons from a metal

You can calculate the amount of charge it acquires by

$$Q= nq = 100 times 1.6 times 10^{-19} C = +1.6 times 10^{-17}C$$

which is positive since it lost electrons and $q$ is the charge of an electron. The voltage can be obtained from the electric field

$$Delta V=E Delta x$$

where $V$ is the voltage between two points separated by a distance $Delta x$ along the filed. Using Maxwell's equations we can compute the electric field (outside) the ring to be

$$E = frac{Q}{4piepsilon_0}frac{x}{(r^2 + x^2)^{frac{3}{2}}}$$

where $r$ is the axial distance from the centre of the ring and $epsilon_0$ is the permittivity of a vacuum. This will not work if you consider the electric field at any point which is not on the axis. And if $x>>r$ this expression will look like the electric field due to a point charge.