I read a report on Erowid here: Datura Stramonium - Erowid Exp - 'Useful as an Anti-emetic'

Here's what he says in that report

He claims that 10 seeds are effective at treating motion sickness without much side effects.

In Principles of clinical toxicology By Thomas A. Gossel, J. Douglas Bricker, page 252, they state that 10 Datura Stramonium seeds contain approximately 1 mg of atropine. They state that 4-5 grams of the leaves or seeds can be fatal.

EDIT: The information in Principles of clinical toxicology  is not correct. The seeds of Datura Stramonium contain 0.1-0.7% alkaloids which are mostly hyoscyamine with lesser amounts of scopolamine and only traces of atropine. I’ve seen over a dozen references that state that as a fact. Most state that the average alkaloid content of the seeds is more likely to be between 0.2 and 0.4%.

I weighed 100 Datura Stramonium seeds from several different vendors and found that the seed weight varies from 5-10 mg with the average seed weight being 7.78 mg. So 10 large seeds would weigh about 100 mg. At a maximum of 0.7% alkaloid content, we’re talking a maximum of 0.7 mg (700 micrograms) of alkaloids for 10 large seeds, not 1 mg as stated in that book above. Also that 0.7 mg would be mostly hyoscyamine and not atropine. Even so, the maximum safe dosage for hyoscyamine is 1.5 mg (1500 micrograms). For that amount you would need 21 large maximum potency seeds.

Many references I’ve found set the maximum safe dosage of Datura stramonium seeds at 10 seeds which contains roughly half the maximum safe dose of hyoscyamine.