The current limit is $18k of personal, tax deferred contributions. That means if your employer matches up to $6k, you can contribute $18k and have a total annual contribution of $24k.
Also, the maximum contribution limit is $53k/year. Of this, only $18k can be pre-tax. The IRS allows you to convert any post-tax money in your 401(k) into a ROTH-IRA in what's become known as the
mega-backdoor ROTH. If you have the ability, using a mega-backdoor ROTH is an amazing way of goosing your retirement savings and never having to worry about taxes in ER.