For hydrolysis to monosaccharides, one molecule of a disaccharide needs only one molecule of water.
C12H22O11 (sucrose) + H2O = C6H12O6 (glucose) + C6H12O6 (fructose)
Structurally, a disaccharide molecule may be viewed as a product formed by the condensation of two molecules of monosaccharides with the elimination of a water molecule. So, only one H2O molecule is needed for the reverse process.
