We know that the equation is balanced because the number of zinc, hydrogen, sulfur and oxygen atoms are the same in both sides of the equation.
Zn + H2SO4 → ZnSO4 + H2
We can see that there is one molecule for Zn and also one molecule for ZnSO4 (we know this because there are no numbers before the formulas, so the 1 is implied)
That is why the mole ratio of zinc to zinc sulfate is 1 : 1
