18's a bit thick, I'd personally start with 20 making components that will keep their shape, but it depends on what you are making, and how strong your fingers are!
Have you seen these pages:
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they're quite useful.
Also are you working in sterling silver wire or plated etc. If it's plated hammering it's not necessary as it wont help harden the wire shapes.
Depending on what you make and how much you work the wire will affect how well your components keep their shape.