What's the point of growing so much heart-shaped herb if they're not going to store any or have more people ingest it?

There are a few benefits/reasons to cultivating such a robust garden of a single plant that's not harvested so often.

In case of disaster.

The first and most relevant reason would be in case of a disaster. By having an abundant amount of plant that's spread across a decent sized area, there stands a better chance of a few plants surviving.

And indeed, this turns out to come in handy during the events of Black Panther. If the Wakandans had instead grown just a small patch of herb, then, with no doubt Killmonger would have destroyed all of the heart-shaped herb, and T'Challa wouldn't have become the Black Panther again.

Outside of the events of Black Panther though (since only a single heart-shaped herb survived and was later ingested), generally speaking, any surviving plant could then be used to assist in regrowing the garden.

For better health and higher potency.

The second benefit is that, by promoting abundance in your garden, you also get a higher degree of genetic variation amongst the population. And so, this has the benefit of being able to select for plants with the highest potency, and with the most desired traits, to then produce future generations.

Out of respect and tradition.

The heart-shaped herb has spiritual and religious connections with the Wakandans. By growing a large, healthy and robust garden of this plant, they're showing respect to their ancestors, and to their current king.