  There is a debate as to what is the value of the the entropy $S$ of extremal
black holes. There are approaches that yield zero entropy $S=0$, while there
are others that yield the Bekenstein-Hawking entropy $S=A_+/4$, in Planck
units. There are still other approaches that give that $S$ is proportional to
$r_+$ or even that $S$ is a generic well-behaved function of $r_+$. Here $r_+$
is the black hole horizon radius and $A_+=4\pi r_+^2$ is its horizon area.
Using a spherically symmetric thin matter shell with extremal electric charge,
we find the entropy expression for the extremal thin shell spacetime. When the
shell's radius approaches its own gravitational radius, and thus turns into an
extremal black hole, we encounter that the entropy is $S=S(r_+)$, i.e., the
entropy of an extremal black hole is a function of $r_+$ alone. We speculate
that the range of values for an extremal black hole is $0\leq S(r_+) \leq
A_+/4$.
