0
This is the principal quantum number, n.
principal quantum number
The principal quantum number refers to the electron shell, or energy. Refer to the related link to read a Wikipedia article about quantum numbers.
it means the major level of orbital
like 2S1, the 2 is the quantum number
3D4, the 3 is the quantum number
The principal quantum number n = 3 and the azimuthal or orbital angular momentum quantum number would be l =1 .l = 1