Achillea millefolium, also known as western yarrow, has 1418947 seeds per pound. It thrives in mildly disturbed soils of grasslands and open forests.  
Seed treatment reccommendations: 30-60 days cold-moist stratification. 
More Info: USDA Plant Guide: https://plants.usda.gov/DocumentLibrary/plantguide/pdf/cs_acmi2.pdf