That depends on your body.
SWIM can feel the effects from just one Hawaiian Baby Woodrose seed on its own, but some people can't.
3-6 Datura stramonium seeds will increase the potency by about 1.5-2x. It's a very nice combination. It gets rid of most of the stomach distress caused by the seeds and increases the visuals and makes the trip more LSD-like. It also makes mescaline more LSD-like for SWIM.
Hyoscyamine, the main active alkaloid of Datura stramonium seeds (Jimson weed), is both a stimulant and a sedative, having slight sedative effects on your mood, calming you down, while having slight stimulant effects on your mind and body.