Answer
$M$ is the only midpoint for $\overline{AB}$.
Work Step by Step
If $M$ is a midpoint of $\overline{AB}$, then $AM$ = $\frac{1}{2}AB$.
Assume that $N$ is also a midpoint of $AB$ so that $AN$ = $\frac{1}{2}AB$.
By that substitution, $AM$ = $AN$.
By the segment-Addition Postulate, $AM$ = $AN$ =$NM$.
Using substitution again, $AN$ + $NM$ = $AN$.
Subtracting Gives $NM$ = $0$.
But this contradicts the Ruler Postulate, which states tha the measure of a line segment is a positive number.
Therefore, our assumption is wrong and $M$ is the only midpoint for $\overline{AB}$.
