smc

Let $a$ be the smallest element, so $a+18$ is the largest element. Since the mode is $8$, at least two of the five numbers must be $8$. The last number we denote as $b$. Then their average is $\frac{a+(8)+(8)+b+(a+18)}{5}=12⟹2a+b=26$. Clearly $a\le 8$. Also we have $b\le a+18⟹26-2a\le a+18⟹8/3<3\le a$. Thus there are a maximum of $6$ values of $a$ which corresponds to $6$ values of $b$; listing shows that all such values work. The answer is $\boxed{6}$.