Mole ratios and limiting reagents
The coefficients in a balanced equation are mole ratios, and the reactant that yields the least product is the limiting reagent.
A worked example
For N2 + 3H2 to 2NH3, 4.0 mol N2 could make 8.0 mol NH3 but 3.0 mol H2 could make only 2.0 mol, so hydrogen limits the reaction.
The mistake people actually make
Do not pick the limiting reagent by which amount is smaller in moles or in grams. Convert each reactant into product and compare those two figures.