Unless you replaced major components or had a sudden, catastrophic release of refrigerant and oil from something like a burst hose you don't need to add additional oil. Very little is lost during normal evacuation of the AC system.
Given that the car is 13 years old you should consider replacing the drier, in which case you would add about an ounce of PAG oil of the viscosity specified for your AC system. (That's a "rule of thumb" estimate to keep the oil level close enough for government work.) It's also a good idea to add dye when charging in order to help find any current or future leaks.