IMO plant more than you plan to grow, better to have more and a choice/selection in the most healthy plants. Some may not germinate, some may be genetically superior and more vigorous than others. I say plant more than you need so you have plenty to work with in case something goes wrong.
for 12 plants i'd grow say 15-20 female seed's ,this allows for any none germ's or weak plants. pick the best 12.
for regular seeds do at least double what you need, so 25 -30 seeds, then select out any males and weaker plants.
you could always just pop 12 and cross fingers and toes.
I started out growing 1/2 pks. but before long I'd been stock-piling beans and realized I should just go for it.
I haven't bought seeds in yrs, being busy w/ hobby breeding and swapping gear w/ like-minded friends.
Now I pop as many as I can, get ruthless in thinning them out. Getting the best plants possible is my #1 priority when growing from seed, and the more you can look at, the better. Also makes you a better grower, learning what to look for @ as many plants as you can.