How do you balance N2O5 plus H2O yields HNO3?

N2O5 + H2O == HNO3 + HNO3

Usually written as N2O5 + H2O = 2HNO3

You have 2 x nitrogen in N2O5 , so 2 x nitrogen in the products, one each in the two molecules of nitric acid.

You have 2 x hydrogen in H2O , so 2 x hydrogen in the products, one each in the two molecules of nitric acid.

You have 5 oxygens in N2O5 + 1 oxygen in water making for 6 oxygens. These oxygen are equally shared ( 3 each) in the two nitric acid molecules.

However, we do not write ' HNO3 + HNO3 ' in chemistry , but use a little bit of 'shorthand' and write it as ' 2HNO3 '. to indicate that there are two(2) molecules of nitric acid formed.