Since the plant is destroyed when it is harvested, they would have to grow new ones. I assume they use seed but I am not a cannabis grower so I am unsure about their cultivation practices. My understanding of what I've read is that it grows fairly fast and growers can do more than one crop in a full year. How many exactly, I don't know.
Let me contribute a bit to the cannabis side of this. I'm currently lab director for a company growing cannabis indoors and using it for DEA legal, FDA approved drug development. The cannabis is initially started from seed. But once plants are identified with the desired characteristics, we establish mother plants and start additional grows from cuttings. We use tissue culture to archive the genetics from the desired plants so we can restart a few years down the road when the mother plants have been exhausted.
You can keep cannabis growing for a long time by managing the light cycle to keep them from flowering. But once you move to flower it's a one way ride and we destroy them after harvest. Our plants are grown in rock wool substrate with a constant low supply of chemically blended fertilizers. The substrate, water, chemicals and plants are all tested for pathogens, heavy metals, etc before we start. We use no pesticides since the products are intended to make drugs.
With all this, and gowning, and restricted access we still sometimes get bugs and fungus. If you're growing indoors you WILL get bugs and fungus. Plan on washing you plants down in the shower a couple of times through the winter..