"Only one-tenth of a single percent of DNA (about 3 million bases) differs from one person to the next. Scientists can use these variable regions to generate a DNA profile of an individual, using samples from blood, bone, hair, and other body tissues and products."
From this I'd say about 3,000,000,000 pairs of bases. Incidentally, a DNA strand stretched out is approximately 6ft long. DNA is in every cell in the body which has a nucleus, which is pretty amazing...