100g of fresh duckweed has been found to contain 18μg of vitamin B12. In this case, the company was growing lemna minor and wolffia, seemingly in outdoor tanks exposed to the air.
Public Health England recommends 1.5μg of vitamin B12 per day, or the amount that would then be in 8g of fresh duckweed. Dried duckweed should be around 10% the weight of fresh, and B12 seems resilient to drying.