Do lots of people get prostatectomy, or is it unusual?

Lots. Treatment of prostate cancer should be tailored to the patient. There is no right and a few wrongs. Prostatectomy is good if you have >10 years to live and are comfortable with the risks vs benefits. For more info @thepeedoc www.Peedoc.
1/6 have cancer. Surgery and radiation are the only 2 cures. Determining who needs a cure and who can just "watch" their cancer is criticial. No perfect test exists to do this. Psa, despite its controversy, is a useful screening tool. Newer tests are appearing to help psa be better. Surgery, in expert hands, gives better functional recovery than ever before.
Depends on cause. Some of the common cause for prostatectomy are benign prostatic enlargement causing urinary retention, nodules in prostrate on radio imaging, or suspected malignancy( cancer ) on biopsy.