For our DS only 1 out of 3 MOCs interviewed. However, remember that interviews are at the discretion of the MOC. For example, DS went through the process when Dole was a Sen. She didn't interview, but Burr did. She is no longer a Sen., which means you could get out of date info.
Additionally, just because the MOC has done it previously doesn't mean they will do it this yr., and conversely if they never did it, doesn't mean they won't now.
The other part of the equation is that some MOC's talk, and when given the opportunity to interview they state, if you have a nom., you will not be interviewed...in other words, they most likely will not be placing you on their slate.
My best piece of advice is not to worry about the stats of how many do or don't and look at your area in particular.
As a long time poster and a mod. I am sure you know it will start lighting up like a christmas tree when the interview letters start coming out.