The number of valence electrons that an element has primarily determines the number of bonds it can form and its reactivity. However, if you are looking for just one response to fill in the blank, the most direct connection to valence electrons would typically be the number of bonds it can form.
If the answer must reflect how valence electrons affect chemical behavior as a whole, then its reactivity could also be a valid choice, since elements with similar numbers of valence electrons often have similar reactivity patterns.
For your question, I would suggest: the number of bonds it can form.