It appears that the only major outlier of this set of numbers could be is 18 but that is incorrect. The answer is none.
An outlier in a distribution is a number that is more than 1.5 times the length of the box away from either the lower or upper quartiles. Specifically, if a number is less than Q1 - 1.5×IQR or greater than Q3 + 1.5×IQR, then it is an outlier.
Basically 18 appears to be inside the box and so does 1