Structural equivalences between atoms of a parent molecule reduce the number of positional isomers that can be obtained by replacing those atoms for a  different element or group.  Thus, for example, the structural equivalence between the six hydrogens of ethane  means that there is just one structural isomer of ethanol, not 6.  The eight hydrogens of propane  are partitioned into two structural equivalence classes (the six on the methyl groups, and the two on the central carbon); therefore there are only two positional isomers of propanol (1-propanol and 2-propanol).  Likewise there are only two positional isomers of butanol, and three of pentanol or hexanol.