Absolutely different varieties will have different seed counts.
My experience is that the darker the seed portion, the better. If you have a matured flower head that has gone brown and dry, and you pull out the seeds from the sepal cup, any that are beige instead of black will not sprout. I have seen them where they are all black but I don't know that darkness equals viability, just that paleness equals immaturity.
And like rainbowgardener's experience, the viability from collected seed like that is not really high. I'll scatter these collected seeds more thickly, instead of 2-3 to a yogurt cup sort of thing.
If you dead head your marigolds faithfully, you will get a higher proportion of immature seeds. But if you leave the flower to dry up on the plant, you discourage more blooms. I dead head, but leave a few here and there to collect a few days later.