= Solution
At the limiting area, the spherical core radius is $R=\ell=1.672$ nm. Its <aggregation number> is
$$
N_{\rm agg}=\frac{4\pi R^2}{a}=\frac{4\pi R^3}{3v}\simeq55.9.
$$
Above the <critical micelle concentration>, free <monomers> remain approximately at 8 mM, so 42 mM of surfactant units are in aggregates. Thus
$$
\boxed{c_{\rm micelle}\simeq\frac{50-8}{55.9}=0.751\ \mathrm{mM}.}
$$
The concentration is of whole aggregates, not the concentration of surfactant molecules inside them. The estimate neglects <micelle> polydispersity and concentration-dependent growth.
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