Percentiles and Box Plots · Challenge problem

The data $4, 9, 12, 15, 17, 22, 26, 33$ have no outliers under the $1.5 \times \text{IQR}$ rule (quartiles are medians of the lower and upper halves, the median excluded when the count is odd). Some extra values, all equal to $17$, are added to the data. What is the smallest number of added values that makes $33$ an outlier?