The number of valence electrons needed for an element to become stable depends on the specific element and its electron configuration.
In general, elements tend to be more stable when they have a filled valence electron shell. For most elements, the valence electron shell can hold a maximum of 8 electrons, except for the first shell which can hold a maximum of 2 electrons.
The concept of stability is often associated with the octet rule, which states that atoms tend to gain, lose, or share electrons to achieve a stable electron configuration with a full valence shell of 8 electrons (or 2 electrons for the first shell).
Exceptions to the octet rule occur with elements having fewer than 8 valence electrons in their outer shell. These exceptions include hydrogen (which only requires 2 valence electrons to achieve stability) and elements in the first period of the periodic table (such as helium, lithium, and beryllium) that can achieve stability with fewer than 8 electrons.
In summary, the number of valence electrons needed for an element to become stable varies, but most elements strive to achieve a full valence shell of 8 electrons, following the octet rule.