One wonders what score they use to screen though....
One logical progression would go like this. Not all programs do this, but I know of several who do.
1. Establish a list of "must interview" candidates based on who they are familiar with, and who they like.
Most programs interview all home candidates. Most medium/small programs interview all who students who rotate through the program out of courtesy, unless you are really bad. In my experience, a lot of the really big name places don't always do this. Same goes for research experiences. If a certain powerhouse research program has 70 students and researchers apply in the same year, they can't interview them all.
2. After establishing the list of must interviews (I would estimate somewhere around 25%-40% of the total number of candidates interviewed), then the programs start to filter out based on step scores, AOA Status, Class Rank and location.
This process would explain the phenomenon of candidates matching with extremely low Step 1 scores (<190)
As for the exact number, after speaking with a few insiders, 240/99 seems to be common, however I did hear of a program up north screening out with 250's. However, it's also common for such programs to not find enough suitable candidates at one particular score plateau and subsequently lowering it. One other north/midwestern program made a big point of stating in their letter that they didn't use single item to screen out candidates.
One thing is certain though, the number will continue to increase. In the last 5 years the national average step 1 score has increased by 10 points. The national average step 2 score has increased by 15.