You may not need any spacers or anything else other than the cassette and lockring. The cassette comes with it's own internal (sprocket to sprocket) and most include lockrings.
However, you may (MAY, not WILL) need a 2mm or so spacer behind the cassette to bring it out so it overhangs the frone of the freehub and th lockring can compress it without bottoming against the freehub. This varies brand to brand, and I never sweat about it, and deal with it if/when I need to.
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