Question

How can I know the relative number of moles of each substance with chemical equations?

Answer 1

The relative numbers of moles (called the stoichiometry) is given by the numbers in front of the chemical formulae in the equation - the numbers used to balance the equation.

For example, the reaction of sodium with oxygen is represented by the balanced equation:
4Na + `O_2` --> 2`Na_2O`

this equation reads "four moles of sodium react with one mole of oxygen to produce 2 moles of sodium oxide". Of course you can now use these relative numbers of moles to scale up or down any given quantities e.g. 0.2 moles of sodium will produce 0.1 moles of sodium oxide, or 20 moles of oxygen will react with 80 moles of sodium...