Does a chemical reaction change the number of atoms?

In a chemical reaction, the number of atoms of each individual element remains conserved and are unchanged. The atoms may be rearranged, but their number stays constant. In comparison, in a nuclear change, the atoms of one species may change into that of another. Thus conservation of mass is applicable in case of chemical reactions. This is the basis for writing balanced chemical equation, in which the number of atoms of each species are same on both sides of chemical reaction. 

For example:  `2H_2 + O_2 -> 2H_2O`

In this chemical reaction, oxygen and hydrogen combine to form water. This reaction represents rearrangement of atoms (combination of hydrogen and oxygen atoms in 2:1 ratio). The number of hydrogen atoms are 4 on both sides of the reaction and that of oxygen is 2.

Hope this helps.