Electron in an atom is characterized by four quantum numbers. Number of electrons with  l=1, m=0  in  Cr (Z= 24) atom are

l=1 means p-subshell.Cr- 1s22s22p63s23p64s13d5. Number of p- electrons are 12For l=1, m=--1,0,+1 ;Thus of the 12 p-electrons only four electrons(2 in 2p and 2 in 3p) will have, m=0