How do they know that ml quantum number is half that of the # of elements in its respective block?

[Cassidy Cheng 1J]

For example, in the p block, there's 6 elements and it has 3 orbitals. I understand that there's 2 electrons per orbital and so that's why they add up to be 6 electrons (1 extra electron for each element as you go from left to right in the p block), but how did they know that the ml quantum number has 3 orbitals for the p block? Did they find out just based on experimental data and realized that it matched the 6 elements in the p block? Thanks in advance!

[Claire_Kim_2F]

I do not think we need to know this but I do assume it is from experiments that they realized that.

[SavannahScriven_1F]

They knew ml had 3 orbitals for the p block because ml gives the orientation of the p shape. The p shape is a 3D figure with 2 lobes (like a 3D infinity sign). This 3D shape can be arranged along the x plane, the y plane, or the z plane. This gives rise to p_x, p_y, and p_z. Each of these is an orbital.
So in short, they knew it was 3 because they observed 3 possible orientations(m_l).