how many valance electrons to become stable
Atoms become stable when they have achieved a full outer electron shell. The number of valence electrons needed to achieve this stability depends on the element.
For example, elements in Group 1 (such as hydrogen and lithium) only need 1 valence electron to become stable, as they only have 1 electron in their outermost shell.
Elements in Group 2 (such as beryllium and magnesium) need 2 valence electrons to become stable, as they have 2 electrons in their outermost shell.
Elements in Group 13 need 3 valence electrons to become stable, Group 14 needs 4, Group 15 needs 5, Group 16 needs 6, Group 17 needs 7, and Group 18 (known as the noble gases) already have a full outer electron shell and are already stable.