The answer is 10 O2, it should de 13 O2.
To know if a reaction is well balanced between the reactants and the products, you have to calculate the number of atoms of each element on both sides of the reaction and to check if they are equal.
*For carbon, on both sides, there are 8 carbon atoms.
*For hydrogen, on both sides there are 20 hydrogen atoms.
*For oxygen, there are 20 oxygen atoms in the reagents and 26 in the products, so it is necessary to increase the number of oxygen in the reactants (without touching the others). For this we put 13 O2 in the reactants instead of 10, to have 26 oxygen atoms on each side.