  Elasticity theory calculations predict the number N of depressions that
appear at the surface of a spherical thin shell submitted to an external
isotropic pressure. In a model that mainly considers curvature deformations, we
show that N only depends on the relative volume variation. Equilibrium
configurations show single depression (N=1) for small volume variations, then N
increases up to 6, before decreasing more abruptly due to steric constraints,
down to N=1 again for maximal volume variations. These predictions are
consistent with previously published experimental observations.
