Negative numbers are real numbers. The set of real numbers includes all the numbers that can be shown on a number line including both positive and negative numbers.
If a number can be represented as a decimal, it is a real number. This includes both positive and negative whole numbers, repeating decimals and irregular decimals that go on forever without repeating. For example, -4 is a whole number because it can be represented by -4.0, and 1/3, which can be represented by the repeating decimal .33333 where the threes continue forever, is also a real number. There is an infinite number of real numbers.