Why my 20days newborn have pimples from last two days ?

Probably normal: It's likely normal, but can't say for sure without a photo. Neonatal acne is very common and occurs in 2-3 week-old infants, typically on the face. It'll likely go away on its own within 1-2 weeks. However, if it looks infected (red, warm, painful) or you start to see thicker yellow crust (not just whiteheads), then you need to see your doctor.

Patience: Baby acne is a common and temporary problem.It usually appears by age 2 weeks and disappears by 3 months. Keeping the skin clean is the major plan. If it persists after 3 months and or intensifies, some normal ace treatments can be beneficial, but the doc should look into unusual hormonal issues as a cause at that point.