CSS nth-child 1-6, 7-12, etc

You can use :nth-child(6n+1), :nth-child(6n+2), :nth-child(6n+3), :nth-child(6n+4) etc..

References: http://w3.org/TR/css3-selectors/#nth-child-pseudo , http://w3.org/TR/css3-selectors/#selectors (thanks @NayukiMinase)

Also, a very good example is here: Useful :nth-child Recipes - CSS-Tricks

read the explanation how nth-child works here.

you can use 6n+x, where you have to insert appropriate numbers for x.