How many ounces are in a schooner of beer?

If you are referring to the beer drinking vessel similar to a Chalice or Goblet but with thicker walls and a thicker stem then they are typically 16 or 20oz, this is most commonly called a schooner.

There also exists a drinking vessel of 6 or 8 oz that looks like a small water glass that is less commonly called a Schooner.