Monosaccharides have only one carbohydrate unit.
E.g -glucose, fructose and etc.
Two monosaccharide units are attached via a glycosidic linkage to form a Disaccharide.
E.g -Sucrose (glucose+fructose), maltose(glucose+glucose) etc.
Oligosaccharides have 2-9 monosaccharide units.
E.g-Raffinose (glucose+fructose+galactose)
Polysaccharides have many saccharide units and are linked together by glycosidic bond.
E.g- Starch(300- 1000 glucose units), cellulose, etc.