Hard question to answer because it depends on so many things.

How many of each plant do you want to have?
How are you going to space your plants?
How will you feel if the germination is low on some plants and not on others?
How big are the seeds?
What type of seeds are they?
Are you going to plant anything in pots/various size containers?
If you have more than you want will you be able to give some away or attend or have a spring plant swap?
Are you "heavy handed or light handed" with your seed sowing?

I've been WS for 5 years, this coming year will be #6 so from experience I try to plan this way.

I do host an annual spring plant swap, so that does, pretty much, take care of the extra seedlings if I have any.

I also like to cram my beds full of plants and sow with the HOS method as well and make bold statements with my plantings.

I also do quite a bit of pots/containers as well and I cram those full as well.

Generally though, it depends on the seeds I am sowing, what type of plant it is and how many of that plant I want and how I want to plant them.
It will also depend on if the seeds are mixed colors and I'm trying to get as many different colors of the plant as possible.
It also will depend on how large the plant will get and/or how much of a spread the plant will be at full growth.

Since some plants will grow very tall, but not wide, I will plant them in clumps of quite a few plants together to make a bold statement. Sometimes as many as 20 plants all right together. Zinnias are an example of those.

The small orange flowers in this picutre are zinnia old mexico and that is a clump planting of 10 plants.
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Other plants that will grow tall and single stem, like sunflowers, I will just sow enough to be sure that I have the number of plants I want and then some extras just to be safe.

Tiny seeds like petunias, violas, schizanthus, balloon plant, coleus and the like, I just sow those in containers and the more the merrier for me.
I cram them into containers and beds with the HOS method and let them do their thing. And this is what I get

mixed petunias
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nicotiana
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viola
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schizanthus and chinese forget me nots
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There are seeds I have that I know are not good germinators and I just sow all that I have and hope for the best.

It all really comes down to how much of each plant you want, how you plan to plant them, and what your planting style is and how crammed you want your beds to be.

The safest thing is to always so more than what you really truly want to have. That way you can be sure that you have at least as many plants as you want to have.

"Murphy's Law" always comes into play with gardening. You know that if you want 5 of a plant and only sow 5 seeds, you will be lucky to get 1 0r 2 seedlings, but if you plant 10 seeds to be safe that you get the 5 your want, you know that all 10 will germinate.

That's the safest way to sow your seeds. Always so more than you will need to be sure you have exactly what you want.

Fran

I do one commercial pack per milk jug as a general rule. I sow thickly and plant out HOS.

Lets put it this way I know that the petunias I will get at least a 90% germination rate yet with the bells of ireland I will get maybe a 2% therefor I will sow alots more of the bells of irelnad than the petunias. But I also have more colors of the petunias and I want to a color explosion in the gardens.

And I don't mind the extra starts I share with my sisters as well as other people.

Paula

I have 3 years WSing under my belt now, and I'd say ditto the above comments for most things.

I've had lots of verbena bonariensis the past 2 years, loved them. However, they became so invasive for me I've had to give them up and won't sow them again. On the other hand, some things, like balloon flowers, I haven't succeeded in growing one plant in 3 yerars. If I act stupid and try again, I'll definitely sow a whole pack.

And petunias and cosmos are so easy, no-brainers, that I might just sow one jug of them and direct sow more.

As years of experience pass, you live and learn.

Karen

I was originally looking at planting 4000 - 6000 seeds, but now am planning on sowing 10,000 - 15.000 seeds this coming winter.

That will be 50 - 75 sowing flats with my set up (each flat with 28 three inch pots) Each flat should average about 200 seedlings in 28 clumps to plant.

I figure I'll plant out at between 4 and 8 plants per square foot depending on type. and will have enough plants for 2000 - 3000 square feet of garden.

For a smaller garden, I'd still figure on sowing for 4 - 8 plants per square foot. If I had bad germination I'd still be OK, and if I had a bumper crop I either be able to squeeze the plants in or give away a manageable amount.

If I used a lot of mulch or miracle grow I might want to plan on planting out at an average of 1 - 2 plants per square foot.

A square foot can eat up a good number of plants if you are planting close and layering for understory, middle and top bloomers. If I was planting a 1 ft x 3 ft section with sunflowers, zinnias mixed in below, baby's breath, and alyssum, I could easily wedge in 6-8 plants per square foot for a total of 18-24 plants in that small area.

Lynda

I agree with the comments above. You will find what works for you with some experience.

After several years of winter sowing I have found what works for me. For larger seeds/plants in gallon milk jugs I generally sow 9 seeds and end up with at least 6 or 7 germinating, often all 9 germinate. For smaller seeds I sprinkle a pinch which usually results in a full container.

With 2L and 4 inch pots I sow 2-4 larger seeds and a very small pinch of small seeds. Tomatoes planted in 2L get 3 seeds. As I sow many varieties of tomatoes I need only 1 plant and usually have to find homes for 2 or 3 others.

24 oz. bottles get 2 large seeds and a tiny pinch of small seeds.

I do trades, share with friends, and sell plants at the end of my driveway. If I still have too many plants, into the compost pile they go.