When you grow any amount of seeds, a percentage of them won’t germinate, even if you get them from a reputable breeder. Always count on a few not germinating or dying off, or roughly 1/4 of the total you put in the ground.
When growing regular seeds, some won’t germinate and some will have to be discarded because they’ll turn out to be males. With feminized seeds, some won’t germinate, but a higher percentage of them will turn into flowering plants because there won’t be any males.
If you want six total cannabis plants to harvest for buds and are growing from regular seeds, start with about 9 or 10 seeds. Some won’t germinate and some will turn out to be males, and then you’ll want to discard down to the six best phenotypes.
If growing feminized seeds, you can probably start with a couple more seeds, 7 or 8. A couple won’t germinate, and then discard down to the six best phenotypes.
Make sure to always stay within your state’s legal limit of growing plants.