In this case you have 3 Oxygen.
Consider that H is hydrogen and doesn't have a number on its side so is 1.
N is nitrogen and as well as hydrogen doesn't have a number on its side so is also 1. On the other hand, O oxygen does have a number which is 3. Which means that to make Nitric Acid you need 1 hydrogen, 1 nitrogen and 3 oxygen.
If the number where to be on the left side of the thing and teh start mostly then that would mean that there are the # atoms of nitric Acid... Hope it help :3