How many electrons in an atom can share the quantum numbers n= 2, l = 1, and m subscript l= -1?

Answer is: two electrons.
n - principal quantum number, indicating relative overall energy of each orbital, n=1,2,3,...
l - angular momentum quantum number, shape of the orbital, l = 0,1,...,n−1.     
l =1 for the p orbitals.
ml - magnetic quantum number, ml = 0,±1,...,±l.
This is 2px orbital, in every orbital there is two electrons with different spin.

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