Question

In an atom of barium, how many electrons have l = 0 and ml = 1.

Answer 1

L refers to the subshell, and L=0 goes with the S-subshell. since Ba is in the 6th shell, there would be 12 electrons in the S-subshell. as for ml values, they rage from -L to L, and since your L value is 0, the only ml value you have is 0 and so there are no electrons that have both values of L=0 and ml=1.