It's a difficult thing to define because when do questions spill over into doubts and when do doubts spill over into unbelief? It's a long continuum that most people leaving finding themselves travelling along over a period of time. Some people can even go back and forward in their mind as circumstances and focus change.
Plus it's even more complicated than a continuum because there is an infinite variety of accommodation and negotiation among JWs and their acceptance and rejection of Watchtower truth claims. Some JWs even think the GB are not hardline enough (one elder told me blood fractions should not be allowed and the GB made a mistake) which is a different sort of mental deviation than we more commonly refer to. Others might quietly reject the flood, accept evolution, not practice shunning, celebrate birthdays, not dontate any money or actively encourage others to join, yet in some sense still adhere to the religion. Are they mentally out, or somewhere in beteeen?
There also seem to be a number of abusers who cynically use the religion to abuse others. These people are also "mentally out" in a sense, although not in any sense we can readily identify with or approve.
So it's altogether a very complicated concept and of course impossible to quantify. Except perhaps to say that, in some difficult to define sense, the number of people who are "mentally out" appears to be growing.