Answer is: the coefficient of Hg will be 1.
Balanced chemical reaction: Hg²⁺ + 2Ag⁰ → Hg⁰ + 2Ag⁺.
Oxidation half reaction: Ag⁰ → Ag⁺ + e⁻/×2; 2Ag⁰ → 2Ag⁺ + 2e⁻
Reduction half reaction: Hg²⁺ + 2e⁻ → Hg⁰.
Mercury cation gain two electrons and become mercury metal with neutral charge (reduction), silver lost electrons and become silver cation (oxidation).
