A mole ratio consists of whole numbers because atoms and molecules react as whole particles.
If we have a reaction such as
N₂ + 3H₂ → 2NH₃
we know that 1 molecule of N₂ reacts with three molecules of H₂. The molecular ratio is 1:3.
If we multiply everything by Avogadro’s number N, we find that N molecules of N₂ react with 3 N molecules of H₂. The molecular ratio is N:3 N = 1:3.
But N molecules = 1 mol. Therefore
1 mol N₂ reacts with 3 mol H₂. The molar ratio is 1:3.
The molar ratio consists of small whole numbers because it is the same as the ratio of the numbers of reacting molecules.