Chemical bonding and reactions
Metals lose electrons to non-metals in ionic bonding while non-metals share them in covalent bonding, and every reaction conserves mass.
A worked example
In Al2(SO4)3 the 3 multiplies the whole bracket, giving 2 aluminium, 3 sulfur and 12 oxygen atoms, 17 atoms in all.
The mistake people actually make
A number in front of a formula multiplies every atom in it, a small number after a symbol multiplies only that atom, and a number outside a bracket multiplies everything inside. Mixing the three up is the usual source of a wrong atom count.