About 7,000 a day, though it won't hurt to do 10,000:
[T]wo studies, which, together, followed more than 10,000 men and women for decades, show that the right types and amounts of physical activity reduce the risk of premature death by as much as 70 percent.
But they also suggest that there can be an upper limit to the longevity benefits of being active, and pushing beyond that ceiling is unlikely to add years to our life spans and, in extreme cases, might be detrimental.
[A]t 10,000 steps, the benefits leveled off. “There was a point of diminishing returns,” said Amanda Paluch, an assistant professor of kinesiology at the University of Massachusetts Amherst, who led the new study. People taking more than 10,000 steps per day, even plenty more, rarely outlived those taking at least 7,000.
Both studies pinpoint the sweet spot for activity and longevity at somewhere around 7,000 to 8,000 daily steps or about 30 to 45 minutes of exercise most days. Doing more may marginally improve your odds of a long life, Dr. O’Keefe said, but not by much, and doing far more might, at some point, be counterproductive.
I get about 13,000 per day, in part because of Cassie. Which seems fine, according to the report. Note that neither study actually found a causal link between steps and health; the effects only appear related.