Unless your work limits it, you can contribute the same amount to a Roth 401k as a traditional 401k. The aggregate limit for both is $18k this year and $18.5k this year before employer match (up to $53k with match)
18.5k NEXT year (barring any change by congress)
My work doesn’t have a Roth option. I put the max in my personal Roth and the max in the conventional.