As needed. There is no specific recommendation for routine ultrasound exams during a normal pregnancy. However, most women are going to have at least one ultrasound at 18 to 21 weeks, the ideal time to evaluate for normal fetal structures. There will likely be additional sono's if there is a problem with the size of the uterus (abdominal measurements), gestational diabetes, elevated blood pressure, etc.
Depends. Most women in the western world get 3 prenatal scans: at 12 20 and 32 weeks for dating anatomy and growth respectively. High risk cases (diabetes mellitus; hypertensive disorders; obesity; lupus; multiples; thyroidopathies; smoking; amniotic fluid aberrations; fetal anomalies; vaginal bleeding or chronic placental abruption) merit additional growth scans at 28 or 36 weeks. Your ob/mfm will decide.