I want to know what are the chances of losing a nipple after an FTM top surgery?

You pose a really interesting question. I am sorry you had a complication - no one likes complications and on FTM top confirmation surgery nipple necrosis is really, really frustrating because the whole goal of the surgery is to make you look and feel great even without a shirt on.
Having said that, there is no easy answer for your question. Blood supply to the nipple after surgery can be reduced by many factors: size of breast and nipple, but also position of the nipple, overall health, smoking status, technique used, and so on. 
MTF breast surgery can be specially high risk because it is not just a breast reduction. In breast reduction cases we leave plenty of breast tissue behind the nipple, which gives it blood supply. In MTF top surgery we want both: we want to remove as much breast tissue as possible (to assure it is flat) but we also would like to leave plenty of tissue behind (to assure the blood supply). It ends up sometimes being choice between a rock and a hard place. Some patients, specially if the risk factors are stacked against them (ie larger, low hanging breasts, large nipples that require incision all around it to reduce its size, smokers, etc) may have over 50% risk of nipple loss. Other patients (small breast, patient ok with a little residual fullness that would require staged revision, non smoker) may have a risk closer to 1%. For patients with high risk for nipple loss, the best is to have a discussion ahead of time and see if they want to take the risk or instead choose nipple grafting or a staged operation to reduce the risk for nipple necrosis. 
I hope this answer is helpful and that things get better soon.