Working out how many seeds to buy starts with how many plants you actually want to finish. Some seeds will not sprout, a seedling might damp off, and one plant could turn out weak, so ordering exactly the plant count leaves no margin. For a planned grow of three plants, I bought a five pack from https://budweedseeds.com and kept the extras stored in the fridge. Legal plant limits still apply, so spare seeds should stay seeds until they are needed.