how many faradays are required to produce 21.58g of silver from silver nitrate solution

Dear student,

As we know that : Ag⁺ + e^- $\stackrel{}{\to }$ Ag
  Now , 108 gm of Ag deposited on passing 1 F electricity
  Then , 21.58 gm of Ag deposited on passing ( 1 F / 108 ) x 21.58  =  0.2 F 
  Regards