In my heart, I'm gonna say all can be tamed. In my head, I know that cannot be true. However, I have managed to tame *every* feral cat that has crossed my path. This is not to say it doesn't come without challenges.
First, all of the adult ferals I have tamed...I did so *outside* with absolutely NO constraints on the kitty until it allowed me close enough to touch, pet and hold. After the allowed me to handle them outside, I then began to introduce them to the inside cats via holding the back door open 2" for nose-sniffs and some footsie. When there was no more hissing, then I allowed them to poke their head inside and/or come in, if they wished.
The longest I have had to work with a feral was Pretty (Pretty Green Eyes, from my husband saying "that pretty green-eyed cat is here again"). I trapped her first litter to tame/socialize and they were all (4) adopted to homes in pairs. Then I TNR'd Pretty. Unfortunately, she was hugely pregnant and the cat association I work with spays all ferals because they have not had good success with trying to keep a feral mother contained to birth/raise her litter in captivity. I do not know if they were delivered C-section as they spayed her and hand-raised or if they were aborted, and I really don't want to know. Anyhow, it took 14mo from the time of TNR before she would let me *touch* her fur. Not even a real 'pet' along her fur, just a slight brush of my finger on her fur before she would scoot/scurry out of reach.
To make Pretty's long story, short: It took about 4yrs to turn her from a flighty feral into a calm and relaxed housecat that my husband or any visitor can handle. In my experience, Pretty has been the most challenging. Every other feral was comfortable with being handled after 5-8 months of working with them. Adult cats *will* take more time and patience. Kittens are easier/quicker, however there is a "cut-off" age for kittens that most rescues will not accept past...but I have taken older kittens and successfully tamed and socialized them.
Kittens of about 3-6mo are at the critical age of their life where in nature, it is "do" or "die" and they either become independent and self-sufficient, or they die. ...but that independent and self-suffiecient attitude can make taming and socializing much more difficult to work through the barriers the kitten/cat has put up to protect itself and survive.
Here is a link to some information I have put together about how I work my foster cats. I foster friendly kittens, poorly socialized adult and sub-adult cats and completely feral kittens and adult cats. Each requires a little different approach, but the overall theme remains the same: Advance/Retreat. Ask for a 'little more' with each handling session. Do not overload the cat with sensory input. Short and frequent handling sessions are best at first. Allow the cat time between sessions to process what happened while it was being handled and keep building on that. You want to make forward progress and you especially want to avoid any negative encounters that will set the cat back. You cannot (and should not) "force" an animal to do something, but my overall goal is to gently 'show' the cat how things can be better when they have people-interaction to give them confidence within themselves and be able to become happy and well-adjusted housecats.
This requires patience and dedication, but it can be so very rewarding to see a cat finally open themselves up to you. You also need to *read* the cat very carfefully, so you know exactly how far you can go with the cat and how far you can take the cat, with every session. Just keep building on The Good Things and try to avoid anything "bad".
I hope I haven't scared you off!