Question

Atom for how many electrons does the atom have magnetic no is equal to 1

Answer 1

Calcium; Z=20
Electronic configuration: 1s2, 2s2, 2p6, 3s2, 3p6, 4s2
1s, 2s, 2p, 3s, 3p and 4s are the sub-shells involved. 
Each subshell has one orbital for which magnetic qunatum number is zero. Because magnetic quantum has values ranging from -l to +l  where l is the Azimuthal quantum number.
For s-subshell, l=0 ∴ values of magnetic quantum number is 0.
For p-orbital, l=1 ∴ values of magnetic quantum number are -1, 0, 1.
In calcium, 6 orbitals for which magnetic quantum number is zero. So 12 (6*2) electrons whose magnetic quantum number is zero.