The question "How many times ultrasound is safe?" is difficult to answer definitively without more context. Its safety depends on many factors. However, in the context of pregnancy, guidelines suggest a typical number of scans.
Most healthy women receive two ultrasound scans during pregnancy. According to the reference, "The first is, ideally, in the first trimester to confirm the due date, and the second is at 18-22 weeks to confirm normal anatomy and the sex of the baby." The number of safe ultrasounds can also be interpreted as being dependent on whether the ultrasound is medically indicated. If an ultrasound is medically indicated then it can be interpreted as safe.
