“It isn’t 13 whole notes, it’s only 12. An octave and a perfect fourth.”
Just for the record, the interval that describes the range of our National Anthem is a Perfect 12th: a compound interval comprising a Perfect octave and a Perfect fifth.
That does mean 12= 8 + 5, since the two conjunct (overlapping) intervals share a note.
“The whole trick with the SSB is where you start.”
This is true when singing alone. The key of Ab Major is the best compromise when singing in a group.
If you're way out of your range you just sing parts (that was the story for my husband for years - he sang as a child but after his voice changed he thought he couldn't sing, but he can, he's just a very deep bass).
As a 'tiefer Alt' - really a tenor in drag - the lower the better for me.