What purpose do neutrons serve in the nucleus ? What happens if there are too many or too few neutrons?

These nuclei with different numbers of neurons are called isotopes

neutrons' main function is to attract each other and keep the nucleus of an atom intact. a nucleus will become unstable if there are too many or too few neutrons in it.

Explanation:

too many= it will undergo beta decay (the neutrons will changes into proton)

too few= there will be an imbalance between forces which leads to nuclear imbalance