Answer: These grasses will grow up and the tips will bend outward, thus making them wider at the top than at the base. Each year the base will emerge wider than the year before and with time they can grow huge. So the spacing will depend on how fast you want a full look and how much space you like to see between your plants. You could plant extras now and remove them later, in other words.
In a few years, Pampas grass (Cortaderia selloana) would reach about five feet wide, Maiden grass (Miscanthus sinensis Gracillmus) will reach about five feet wide -- it arches outward more than the pampas grass, and the Zebra, another miscanthus, is overall larger than maiden grass and a bit wider still at maybe six feet.
I think six plants will be enough for a good full look in about three years and perhaps crowded in five, but if you are very impatient you might add one more of each. It's a matter of personal preference. Enjoy!
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