It's rare for harvested crops to drop seeds. If you're in desperate need of some rice seeds, I'd suggest going foraging for some rice plants on garden planets. If you use your Matter Manipulator to remove the plant, it'll drop a seed.
I think you may have forgotten to break the plant when harvesting in order to get the seed. Some plants need to be broken and some just need to be picked like carrots in order to get the seed. Otherwise its like jonsey said and you can get an extra seed by random chance albeit rarely.
