Even an amount of dust that most people would call "a lot" doesn't seem to have an effect on the images. Don't dwell on the spot of your lens that has that one dust particle, instead, think of how much of the area of the lens is dust-free!
Lenses are not air-tight and just the action of focusing back and forth (and zooming in and out on a zoom lens) pulls air, and whatever dust is in that air, into the lens. It's inevitable that you will see some dust in every lens.