It is complicated and you need to get professional advice. It has been shown that as little as 81mg. And it comes coated so it won't irritate the stomach. But for you ulcer people you must know that the problem with aspirin is not only when you first swallow it and it sits in your stomach, but also when it gets into the blood stream and comes back to the stomach lining through the blood. That means a coating is not totally protective for ulcer patients.
Many people can benefit from one baby aspirin a day.
Ask your doctor.