ATP stoichiometry of the Na+/K+ pump
Sodium/potassium-exchanging ATPase (EC 3.6.3.9):


  An enzyme from the plasma membrane of animal cells that catalyzes the efflux of three Na(+) and influx of two K(+) per ATP hydrolyzed. 


You can read about the mechanism on Wikipedia:


  
  The pump, after binding ATP, binds 3 intracellular Na+
  ions.
  ATP is hydrolyzed, leading to phosphorylation of the pump at a highly conserved aspartate residue and subsequent release of ADP.
  A conformational change in the pump exposes the Na+
  ions to the outside. The phosphorylated form of the pump has a low affinity for Na+ ions, so they are released.
  The pump binds 2 extracellular K+ ions. This causes the dephosphorylation of the pump, reverting it to its previous conformational state, transporting the K+
  ions into the cell.
  The unphosphorylated form of the pump has a higher affinity for Na+
  ions than K+
  ions, so the two bound K+
  ions are released. ATP binds, and the process starts again.