12 year olds should normally have lost all 20 of their baby teeth at this age. They should have all of their adult teeth, besides the wisdom teeth, erupted.
Adults have 32 teeth counting the wisdom teeth.
So they should have 28 teeth.
In some rare cases children don't develop certain teeth. Also, some children may not lose all of their baby teeth at this age, but in a normal scenario they should have.
Hope that heps =)