Aftertax Contributions - Max Roth 401K Amount?

There's a limit of $23.5k on pre-tax contributions to Traditional 401(k) plus contributions to Roth 401(k). Since you already made $23.5k pre-tax contributions to Traditional 401(k), you can make $0 contributions to Roth 401(k). However, you are not contributing directly to Roth 401(k). Instead, you are making after-tax contributions to Traditional 401(k) (which is not subject to the $23.5k limit), and the converting them to Roth 401(k). There is no limit to the amount you can convert in a given year.