Is there a formula to determine the total number of constitutional isomers?
There seems to be some confusion here : in order to determine the number of stereo-isomers of a molecular formula (say C4H11N) you would need to first know the number of constitutional isomers as you need to know how many stereogenic centers across all the structures.

There are a bunch of existing questions on constitutional isomers (a, b, c, ...) but the summary is : "it is tricky without software for anything more than very small formulae".

There are mathematical formulae for determining the count of constitutional isomers, although the most powerful techniques are non-trivial (such as those based on the Pólya enumeration theorem). To efficiently list a non-redundant set of compounds from an elemental formula is possibly even harder, as it requires a mechanism to avoid duplicates in the output.