I think Ford has said ST's make up about 20% of the take rate for each model year.
I see a butt-ton of Explorers in my neck of the woods. I see mostly XLT's and Limited here. Next up would be the ST's and then Platinum's. The rare ones to see on the road are the King Ranches and Base models. I think i've only ever seen 1-2 base model in person.
For my neighborhood Explorers, there are two XLTs with sport appearance package plus my ST. Neighbor's good friend who is always over their house with their kids has a Limited. One of the soccer mom's at the school drop off drives a red ST. In my work parking lot, two XLT's and my ST.