What matters is the final thickness. For mine I laminate enough carbon fiber and/or glass so the final thickness is just under 3mm (1/8"). A thinner lip is tough on skirts. Figure roughly at least six layers if you're using all 6 oz plain-weave cloth.

For builders like me who are less expert than Dan, remember that the cockpit rim (coaming) is a major stiffening element for the deck in that area. I sit the deck on the hull, and tape them together to keep the deck shape 'locked in' when building the cockpit rim.....making sure to lay plastic or tape in the deck-hull seam to prevent inadvertent gluing of the hull to deck.

If you find that the rim looks a bit dull after final sanding, you can use auto clear coat to bring back some sparkle (if your varnish has an amber hue).

BTW, the relatively few $$ involved in using CF on the rim is repaid many times over in the 'ooh' factor from onlookers. It's also easier to get 'good looking' than using black pigment, IMO. I think it adds a bit of stiffness to the rim, but as Dan says it's not important in a hand layup.