We are given
s = 5.56 x 10^5
Eo = 0.354 V
We are asked how many electrons were involved in the redox reaction.
We use the Nernst Equation
Eo = s log [H+]
Subsituting
0.354 = 5.56 x 10^5 log [H+]
[H+] = 1 mole/L
Assuming that we have a 1 L volume, we have 1 mole of H+ that participates in the reaction which means there is also 1 mole of electron that is involved in the reaction.