I had a radical prostatectomy in which one nerve was removed. I will have follow-up radiation. I am 57. What are my chances of regaining potency?

Poor. Unfortunately, if your entire nerve was removed and you had good function before, most surgeons tell there patient that you have a 50% chance of regaining function. Once you have radiation, the odds will slip even more. Find a urologist who will be aggressive in helping with your sexual rehabilitation after surgery.
It depends. If you had good sexual function preoperatively and a well-spared neurovascular bundle on one side there is a reasonable chance that you can recover sexual function, as age and preoperative function are the strongest predictors of recovery. However, the radiation therapy may negatively impact sexual function, even in men who have not had surgery. Aggressive rehabilitation measures may help.
Prostate cancer pote. Your chances are not very good I am sorry to say after prostatectomy and radiation.Discuss your options with your urologist.Some times drugs like viagra (sildenafil) and Cialis may be some help.
Poor. Unfortunately, majority of men treated with both surgery and radiation have a poor chance to regain complete potency without use of ED therapy (medication, vacuum device, prosthesis). Fortunately, there are solutions available (as listed) so all hope is not lost. Your main priority right now is treating your cancer. Discuss this with your urologist.