I think well over 90% of JWs believe/know that the Watchtower currently teaches some falsehoods. They only have to look at the fact that their teachings have always undergone changes, to conclude that it is likely they will have more changes in the future, which would logically mean that some current teachings are in error.
The problem is that they value loyalty to the organization more than loyalty to truth, so the fact that Watchtower teaches falsehoods is brushed off by them as no big deal because "imperfect men" and "the light gets brighter" and "wait on Jehovah" and "where else can we go?". They hypocritically criticize other religions for the errors they teach, while bending over backwards to concoct all manner of ridiculous rationalizations to excuse the errors that have been taught and continue to be taught by Watchtower.
They also stoop to the ridiculous practice of measuring their errors against those of Christendom and arbitrarily judging theirs to be forgivable based on bias. They tell themselves that their errors are somehow purer than the errors of Christendom so God will turn a blind eye to their errors but punish Christendom for theirs because ... because they use the name "Jehovah", and preach from house to house, and don't believe in the immortality of the soul, and don't celebrate birthdays, and Jehovah has an especially soft spot in his heart for these particular things and as a result he will overlook all their other hypocrisies and errors and injustices on account of them. Because Jehovah plays favorites when it comes to errors, you see. He hates the popular errors that Christendom teaches but he loves the obscure, weird errors that JWs teach.
It all boils down to patriotism. JWs have been brainwashed into becoming blindly patriotic toward their organization, in much the same way that some are blindly patriotic to their country or to their soccer team or to their political party. You cannot reason with such people. Facts don't really matter to them.
I think it's possible that as much as 20% of JWs no longer believe it's the truth, but continue on for purely social and family reasons. Many subscribe to the philosophy that "even if it's not the Truth it's still the best way of life". They use this reasoning to justify continued membership in, and support of, the organization. They are willing to pretend that it's the truth and not rock the boat. They may tell themselves that even if it's not God's organization then it's the least false of all the false religions.