How much maple syrup will 20 gallons of sap make?

Could you please elaborate on the process of converting sap into maple syrup and how much syrup one can expect to yield from 20 gallons of sap? Is there a standard ratio or does it vary depending on factors such as the quality of the sap, the method of processing, and the environmental conditions? I'm particularly interested in understanding the science behind the transformation and the approximate yield we can anticipate.