If it's BCG, people in about their 50s and older usually have about 4 marks, each about 1 centimeter (half inch) in diameter. These remain as keloids.
People now in their 40s or younger have 18 tiny dots instead. This was designed so that they won't remain as keloids, and they hurt less when you get vaccinated. Fresh vaccination marks tend to appear in purple lumps, but as you grow, they turn into skin-color dots. You can hardly notice them on most adults, but some are more recognizable than others.
All people who were registered residents in Japan in their infant days (including those who aren't Japanese) have at least one in one arm. Until the year 2005, you were supposed to get 3 tuberculin tests in total before you reach the age 15. Those who got a negative reaction needed to get a new vaccination. So I suppose the girl is right in a way that those who are "super healthy" only gets it in one arm, although negative reactions don't really mean you're "sick".
According to Wikipedia Japanese version, people around the world are vaccinated against tuberculosis, but Japan is about the only country that uses "stamps" which produce the 18 dots. Scroll down the following site to see a photo of the stamp.