In an atom, the total number of electrons having quantum numbers n=4, |ml|=1 and ms=-1/2 are:

n=4
l=0,1,2,3
|ml|=1 ⇒ ±1
ms=-1/2
For l=0, ml=0
l=1, ml=-1,0,+1
l=2, ml=-2,-1,0,+1,+2
l=3, ml=-3,-2,-1,0,+1,+2,+3
So, six electrons can have |ml|=1 ms=-1/2 (two each in p, d and f subshell).