Thanks! This is a good start to the question, with an important disclaimer.
This video is answering the question of how many seeds to plant per hole, pot, or cell.
I think the statement about large seeds (and their many cells) was a nod to peony seeds. In fact, one cell of a peony seed may have just one seedlet, while many cells may have three or four.
So when you see one or two peony seeds per cell, it's possible that you may have three or four total seedlets per cell. But it's also likely that you may only have two.
When it comes to smaller seeds, there can be an extreme number of seeds per cell. For example, a cell of borage may hold hundreds of seeds.
So it's hard to give a simple answer to this question. You have to look at each seed's individual characteristics.
Understanding that a cell of a peony may have only one seedlet, while the same cell contains many seedlets in borage, you can begin to formulate an answer to the question.
Mind you, both peony seeds and borage seeds will have more seedlings than only two.
Answer One: Seed Germination Rates
The answer to "How many seeds should I plant to grow the best plants?" also has a complicated answer that varies depending on such things as the type of seed and where you live.
You can ask someone in your local gardening club or ask at your local nursery. And you could research the answers on the internet.
If none of that sounds like fun, there's a simple answer that will work in most areas: the rule of 15.
This rule of thumb is where you get the number you want by subtracting 15 percent from 100 percent.
For example, how many carrots should you plant in a 12–pot? First, count 100 percent by making a simple list: 12, 15, 18, 21, 24, 27, 30, 33, 36, 39, 42, 45, -10, -15, -18, -21, -24, -27, -30, -33, -36, -39, -42, and -45.
Then count off 15 percent from 100 percent. That's 85 percent.
To work out the answer, divide 85 percent by the number of holes, which is 12, and the answer is 7. You'll get 7 carrots out of a 12-hole, 12–pot.
(To work out a larger example, use the calculator link to the right)
Answer Two: Seedling Selection
If that answer didn’t satisfy your curiosity, keep in mind that there is a potential for several different plants to emerge from the same seed. This phenomenon is called “sister bulking”. For example, when farmers grow corn, they typically harvest about 150 corn seeds, place them in a seedbed, and plant them 1 inch apart. The seedlings that come up are of different heights, from which seeds yield cobs of different length.
That variation is the result of what’s called “bunching.” Bunching is a naturally occurring phenomenon whereby offspring from the same seed develop, sometimes to different sizes. To prevent bunching in corn, farmers carefully evaluate and select seeds for planting. They usually harvest about 150 of the best seeds each year and plant 1000 seeds per site.
For the most part, when you’re planting seeds for annual beds, you want them to be the same size so they mature at the same time. But you may be interested in how to increase germination rates for a specific type of seed.
Generally, the easiest place to find the answers to your questions is to buy seeds from a seed company. They are always happy to answer questions about germination rates and seedling selection.
Exceptions to The Rule
If you are only starting out and are only growing a variety of seeds for yourself, sample sizes won’t matter much. But if you're growing semi-large quantities, it can matter a lot. If you're not careful, you will have a lot of leftover seeds for the second season, which can result in a lot of money wasted. And while you're at it, you can get seeds that don't germinate, which again will result in a lot of money wasted.
If you're opting for a mix of potting soil, vermiculite, and peat moss, use a disbudding process to remove male seeds (Source). This makes sure all remaining seeds are female, and therefore, should produce good results.
If you have a high germination rate and the ratio of male and female seeds is fairly similar, you may want to save half of them for next season, and choose the best of the best to plant now.
Compromising a bit on the quality of your seeds can also help your budget. Some companies sell more seeds than they should be putting out, and if you are willing to risk it, your seeding rate may not matter as much. You may end up with a few bad seeds, but the end ratio will turn out just fine.