We had to go through an application/interview process as well. My school has 5 sororities - 4 large and one (mine) relatively small - so each of the 4 big houses had to provide 4 PXs, and mine had to provide one.
The year I was a PX, each of the 4 big sororities was asked for 6 candidates of which the Panhel rush committee would select 4. My sorority was asked for 2 candidates of which Panhel would select one, but I was the only one interested that year. (The committee could have rejected me and asked for another application from my sorority if they had so wished, but they didn't
)
So we had 17 PXs for about 150 rushees.