smc

Note that $12.5\%=\frac{1}{8}$, $25\%=\frac{1}{4}$, and $50\%=\frac{1}{2}$. Thus, since the frequencies must be integers, $N$ must be divisible by $2$, $4$, and $8$ (so that $\frac{N}{8}$ etc. are integers), or in other words, $N$ is divisible by $8$. Thus the smallest possible value of $N$ is the smallest positive multiple of $8$, which is $8$ itself, or $\boxed{8}$.