Any object has a self capacitance $C$. This relates the charge on the body to the potential at its surface relative to infinity via the usual capacitor equation:
$$ Q = CV \tag{1} $$
For a spherical object the self capacitance is easily calculated since Gauss' law tells us that the field near the surface of the object is the same as the field from a point mass of the same charge at the centre of the sphere. That is, if $R$ is the radius of the sphere the potential is just:
$$ V = \frac{1}{4\pi\epsilon_0} \frac{Q}{R} \tag{2} $$
And comparing this with equation (1) we see this means:
$$ C = 4\pi\epsilon_0 R $$
This is why when we make an approximate calculation of the charge on a Van de Graaff generator we treat it as a sphere. In principle it is possible to do the calculation for other shapes but this gets complicated because we no longer have the spherical symmetry to help us.