How many ATP molecules need to be converted to ADP to link two glucose molecules?

None. Here we are talking of the synthesis of disaccharides. The biological mechanism entails the reaction known as "condensation", where two monosaccharide molecules, as glucose, join together to form a disaccharide. Condensation reaction involves the loss of a water molecule per two monosaccharides in a chemical reaction. Two OH groups, one from each sugar molecule, come together to release a water molecule and form an oxygen bridge between sugar molecules. One of the OH groups is attached to the anomeric carbon (the carbon that has 2 oxygens bonded to it).