Seeding rate varies from grower to grower and changes greatly based on your personal growing practice and medium of choice, it can also vary from lot to lot of seed (seed size). A good general rule for small seeds (Arugula, Broccoli, Cabbage, Kohlrabi, Mustard, Lettuce...) is 1oz per 10x20 tray. You can increase or decrease based on your preferences after a test grow. A larger seed like sunflower, you'll find yourself using closer to 9 ounces.
For more detailed answers we highly recommend some data from professional growers, such asThe Bootstrap Farmer, or Curtis Stone over atUrbanFarmer.comwho offers classes online!