To determine the number of capsules the nurse will administer, we need to consider the dosage prescribed and the available strength of the capsules. 

The prescription states: cephalexin (Keflex) 0.5 g p.o. qid (four times a day). 

Given that the available strength of cephalexin capsules is 250 mg, we need to convert the prescribed dosage from grams (g) to milligrams (mg) to match the capsule strength. 

1 g = 1000 mg 

0.5 g = 0.5 * 1000 mg = 500 mg 

Now we know that the prescribed dosage is 500 mg, and each capsule contains 250 mg. 

To calculate the number of capsules needed, we divide the prescribed dosage by the strength of each capsule: 

Number of capsules = Prescribed dosage / Capsule strength 

Number of capsules = 500 mg / 250 mg 

Number of capsules = 2 

Therefore, the nurse will administer 2 capsules of cephalexin (Keflex) for each dose.