Some varieties are nearly seedless, maybe out of every 3 or 4 fruits there will be 2 seeds in one of the fruits. Other varieties are completely seedless. Some varieties are considered seedless, but in reality just have a small number of seeds that are very tiny and shriveled up, so you mostly don't notice them. There are several different possible ways breeders go about trying to create seedless varieties, it's usually not easy. In some cases it's possible to grow a seedless variety from seeds, but can often be much more difficult for several reasons.
If you keep growing enough citrus from seeds, hundreds of seedlings, over 2 or 3 generations, you are very likely to encounter a plant that produces seedless fruit. But this is usually too cumbersome to be used as an intentional strategy by breeders. Radiation-induced mutation is another way. Or hybridizing a tetraploid plant with a normal diploid.
