The amount of seed that you use varies depending on what you intend to use the area for.
Lawns (lawn seed)
If you are reseeding (starting from scratch) a lawn, the typical sowing rate is 70 grams per m2. This will provide a good thick lawn, and it should not need over seeding with more grass seed. If you are overseeding (patching up) then a good rate to work on is somewhere between 35-50 grams per m2. The sowing rate for overseeding largely depends on how bad the existing lawn is. If you’re just trying to green the area up 35 grams per m2 is good. If you’re patching up, or filling any large gaps, then 50 grams or more per m2 is sufficient.
Wildflower areas (using wildflower and grass seed mixtures)
In contrast to lawn seed, the sowing rate for wildflower seed is much lower. 2.5 grams per m2 is all you need for a grass and wild flower seed mixture. Because it is so difficult to measure out this quantity, you can mix the seed with some sand to bulk it out, and make it easier to spread.
Wildflower areas (using wildflower seed only)
The sowing rate for just wildflower seed can be anything from 0.5-2.5 grams per m2 depending on the density of flowers you want. It is common to just spread this seed by hand in patches where you are trying to establish wildflower seed.