The term 'Number of Mousquetaires' refers to the number of members in the legendary group of French musketeers. The answer to this crossword is 'TROIS' (5 letters). This is because there were three original members of the Musketeers: Athos, Porthos, and Aramis. The Musketeers were a group of highly skilled swordsmen who served as bodyguards to King Louis XIII of France during the 17th century.