Deschampsia cespitosa, also known as tufted hairgrass, has 1500000 seeds per pound. It can be found on all types of grassland, although it prefers poorly drained soil. 
Seed treatment reccommendations: The species is slow to establish the first year
but can dominate a site by the third year if seeding
rates are heavy (2 to 3 pounds per acre) 
More Info: USDA Plant Guide: https://plants.usda.gov/DocumentLibrary/plantguide/pdf/pg_deca18.pdf