It depends on what you're trying to prevent and what your risk factors are.

For patients with existing heart disease or prior strokes, the NNT is between 50 and 333 depending on the outcome you're considering. Source:

http://www.thennt.com/nnt/aspirin-for-cardiovascular-prevent...

For prevention of a first heart attack or stroke, the NNT is between 1667 and 3000 depending on the outcome. Notable is that no deaths were prevented by the therapy. Source:

http://www.thennt.com/nnt/aspirin-to-prevent-a-first-heart-a...