Depends on whether or not you want to believe Italian hype... as it would be 7/14 depending on the basket used.
For me it depends on many variables... the machine itself as head spacing, basket design, etc. plays a part, as does the coffee being used, fineness, tamp pressure and what you're trying to achieve in viscosity/purity. My average dose is 16-17 grams for a stock Gaggia double basket. Never wasted time toying with singles.