If your doctor recommended saw palmetto for your prostate problem, you should've gotten the recommended dosage from your doctor. Otherwise, you can follow the following dosage recommendations.
For Enlarged Prostate
320 mg daily in one dose or two divided doses
For BPH (Benign Prostatic Hypertrophy)
640 mg daily
Other Suggestions
1 to 2 grams of ground, dried, or whole berries daily
As I said at the beginning, you should consult with your doctor if you are not sure about the right dosage. When you order saw palmetto in a bottle, it will give you general guile lines on how much to take daily. So read the label carefully.