Indeed, each plant will only produce 8 seeds and 8 ressources (if they are of a type that gives a ressource, that is), after the 9 days growing cycle there is a 8 days production cycle (during server downtime), total 17 days. Once those are over, harvest, set to decorative, either cut for reeds (basket weaving) or clipping (plant pigments) and then re-plant new seeds.
Raised Garden Beds automatically take care of the plants (watering, killing parasites, healing diseases) instead of having to apply water/potions yourself, but there is still the normal growth cycle.
If you allow the plants to self-pollinate on the 8th day, you'll have "ID'ed" seeds with the full name of the type. It's on the 8th day that you must pollinate yourself if you want to produce hybrids, more colours etc.
Hope this helps.