Want are chances for miscarriage of one baby with twins?

About 30%: 25-30% of twin pregnancies will reduce to one naturally. In addition there is a 5% risk of loss after 20 weeks. I know that is scary but keep in mind that a normal single pregnancy has about the same risk of miscarriage.

Depends on age: Usually depends on age of the pregnant woman. Risk increases with advancing maternal age. Risk is roughly similar to the risk of miscarriage of a singleton pregnancy, but appears higher since with twins, there are essentially two pregnancies, each with its own risk of aneuploidy (or abnormal chromosal karyotype such as down syndrome).

Depends.: Most of us are probably survivors of multifetal conceptions that spontaneously reduced to single babies over time. So your answer hinges on the time in pregnancy you are talking about. Once a twin pregnancy has been seen by prenatal ultrasound, alomst all twins will continue to thive, however the risk of loss of one twin is higher than for miscarriage of single babies, especially <14 weeks.