For those who are wondering, it's 4 plants a plot, with two plots maximum at the moment. However, there's a clear spot for a third plot in every home instance.
In the Rata Sum and Black Citadel home instances, the garden is right near the entry point of the instance. In the other three home instances, you also get an item near the entry point that sends you directly to the garden.
You start with a good selection of seeds, and have to get the rest by harvesting them out in the wild. Seeds are NOT a sure drop when you harvest the correct kind of plant, so you may find yourself going nuts wondering how all the raspberries in Tyria suddenly became seedless.
Plant the seeds before the daily reset, and you can harvest them after. You only need to change anything after that when you want different plants, otherwise the nodes will stay as you've set them and be harvest-able every day.
And yes, others can harvest from your garden. I don't know if you can harvest from multiple gardens per day, or if it's limited like most resource nodes in the home instance are.
I never did get a purpled out Warshade.