To which set or sets of numbers does the number - 5 belong

Well, the largest set it belongs to is the set of all real numbers - numbers which can be written on the number line. - 5 is of the set of rational numbers since it can be written as a fraction (-5/1 for example). It is an integer since it is a whole number. Thus:

-5 ∈ ℤ (Integers) ∈ ℚ (Rational Numbers) ∈ ℝ (Real Numbers)