There are about 16.9 U.S. fluid ounces and about 17.6 imperial fluid ounces in 0.5 liters. When discussing liquid capacity, the word ounce refers to fluid ounces. There are other units that measure mass simply called ounces.
Density must be known to convert from liters to a unit of mass. Liters measure volume, and to convert to mass, the volume must be multiplied by the liquid's density. For example, the density of room temperature water is around 1,000 grams per liter. The product of multiplying 0.5 liters by that figure is equal to 500 grams. Five hundred grams is around 17.6 ounces. However, if the density is different, the amount of ounces changes as well.