The number that describes the main energy level of an electron in an atom is

the principal quantum number (n)

Explanation:

Quantum numbers are found in the mathematical description given to each orbital (Schrödinger's wave equation). Schrödinger is the 5th atomic model and it seeks to describe the characteristics of all the electrons in an atom. These digits are represented by letters and each of them indicates the position and energy of each of them.

The main quantum number always has integer and positive numbers and allows us to know the energy level of the orbital as well as its size. N determines the measure of the orbital, so the greater the probability of finding an electron near the nucleus of an atone, the energy of the orbital increases. All those groups of orbitals that share the same value of N are known as level or layer.

The values that the main quantum number (n) can adopt can be positive integers such as: n = 1, 2, 3, 4, 5, 6, 7