How many molecules of water of hydration surround each molecule of sodium carbonate decahydrate, na2co3•10h2o

10 Molecules Because the coefficient shows how many molecules are there 

10 molecules of water of hydration surround each molecule of sodium carbonate. That's why it is known as sodium carbonate decahydrate.

Explanation:

When the soluble compound sodium carbonate dissolves in water, the partial negatively charged side of the polar water molecules surround the positively charged sodium ions, while the partial positively charged side of the polar water molecules surround the negatively charged carbonate ions.