Dave, I am afraid you may not find anybody willing to share about that. The topic has regularly surfaced, and either nobody uses that monetization model, or nobody is willing to share.
I would use an indirect method : find apps using the rental model, and look at the number of reviews. That would be a way to estimate the number of subscribers. Unfortunately, the MAS has such a dismal search feature, “rental” brings up all sorts of real estate stuff, and no relevant example of rented software.