no more than 20 seeds and at least $1 per pack

what kind of seeds are they?

Every kind of seed. I save lots of stuff.

Consider the yield of edibles ... 20 radish seeds is not much. 20 squash seeds is OK.

I find most home gardeners don't want more than 20 to 25 seeds even for things like beans and radishes. For things like tomatoes and lettuce, 10 to 15 seeds is plenty. They want to use all the seeds up and most people are doing 10' x 10' gardens so 20 radishes would be plenty for them.

I really can't imagine anyone being happy about getting 10 lettuce seeds in a packet, even for $1. I think many home gardeners direct seed most greens, including lettuce, and expect at least a 1/2 teaspoons worth. Besides, most small seeds are produced on plants in large enough quantity that its still very profitable to sell this much as long as you get a retail price.
Example: This past season I let 10 ft of arugula go and now have to figure out what to do with a quart of seed!

I agree that for larger seeds (beans, pea, squash) 20 or so is usually enough.

Well I am not much into counting so I just grab a pinch of seeds and see how it looks and maybe do another. I sold the packets for $1 or 5/$4. Many people have restrictions on their growing as to small size or only pots or whatever so they didn't have 5 to choose.
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I organized them in these trays and they actually stayed neat throughout the market. I am sure more will sell in the next few months. I did get good feedback from people who appreciate local heirlooms.

I used to sell quite a lot of seeds and the number 1 complaints was too many seeds in a packet and i lost more sales when I generous than when i put few seeds in a packet. remember unlike us market growers most people have tiny gardens are not growing to feed themselves and they generally are growing only a few plants of each type.

You don't want 50 tomato seeds when you are growing 3 tomato plants, or 5 peppers or 10 heads of lettuce, etc.. Ever notice that most commercial seed packets have fewer than 20 seeds in them? There is a reason for that.