if there is no number behind the element, it counts as one, but if there is, you do 1 times x(whatever the exponent is) and thats how many that one element counts for with atoms. then you just add them all up
In order to get HgO you would need 2Hg+1O2=2HgO. Since oxygen is diatomic you need two when it stands alone causing you to need two mercuries to balance out the reactants and the product I hope this helps
