Dear Student,
The structural formula are those having same molecular formula while different connectivity. These are also known as constitutional isomers.
To determine the total possible number of structural isomer there is no formula or trick to identify exact number of isomers.
Thus, to determine the number of possible isomers of given compound, its different arrangement of connectivity to be done and the maximum possible connectivity possible will give the number of structural isomers of that compound.
Regards.