  In this paper, we consider ensembles of affine-control systems in
$\mathbb{R}^d$, and we study simultaneous optimal control problems related to
the worst-case minimization. After proving that such problems admit solutions,
denoting with $(\Theta^N)_N$ a sequence of compact sets that parametrize the
ensembles of systems, we first show that the corresponding minimax optimal
control problems are $\Gamma$-convergent whenever $(\Theta^N)_N$ has a limit
with respect to the Hausdorff distance. Besides its independent interest, the
previous result is crucial role for establishing the Pontryagin Maximum
Principle (PMP) when the ensemble is parametrized by a set $\Theta$ consisting
of infinitely many points. Namely, we first approximate $\Theta$ by finite and
increasing-in-size sets $(\Theta^N)_N$ for which the PMP is known, and then we
derive the PMP for the $\Gamma$-limiting problem. The same strategy can be
pursued in applications, where we can reduce infinite ensembles to finite ones
to compute the minimizers numerically. We bring as a numerical example the
Schr\"odinger equation for a qubit with uncertain resonance frequency.
