How many monosaccharide molecules are needed to form a disaccharide?
It takes two mono-saccharides to form a di-saccharide; you should recognize that the prefix "di" means two.
This isn't really a question, but ... When two monosaccharide molecules join to form one disaccharide molecule, a molecule of water is released. This is called a dehydration (or synthesis) reaction. So in the reverse reaction, when one disaccharide is broken down into two monosaccharides, a molecule of water must be added. This is called an hydrolysis reaction.