How much powder glaze do I need to cover a specific area?

There cannot be an exact answer to this question because of variations in the required glaze/water ratio, in dipping techniques and in the porosity of the ware to be glazed but anecdotally, for example, an average mug takes about 0.15 litres, so 20 mugs would take 3 litres of liquid, (slop) glaze. 1kg of glaze powder makes approximately 1.54 litres of slop glaze so to glaze 20 mugs you will need approximately 2kgs of glaze powder.

See also: Glaze to Water Ratio and Powdered Glaze Preparation.