There are 10 doses of fertilizer in each bottle, but you'll have to keep track of how many you've used once you buy it, since there is no indicator in the game.
Plants produce all of their seeds soon after pollination, and there is no reason to keep them once they have finished their seed production. You will soon become familiar with how long that takes. As you gather the seeds, you can click on the empty seed slot and see whether or not the plant has finished producing seeds. If it has, you can just go ahead and sell it to make room for more plants!
