How do I enter a roth 401k contribution when I keep hitting a combined maximum of 6,500? I see Fidelity says the limit is in the 20k range.

$6500 is the contribution limit for taxpayers under age 50 for tax year 2023 for a Roth IRA. If you are contributing to a 401k this information is part of your W-2 and doesn't need to be separately entered in TurboTax.  The exception for that is if you are self-employed and this a Solo Roth 401(k).  Then the information would be included in the Self-employed pension plan section of TurboTax (rather than the IRA contributions). 

A Roth account in a solo 401(k) is not a Roth IRA.  Nothing about the money you contribute to the designated Roth account in your solo 401(k) is reportable as a Roth IRA contribution.

Contributions from self-employment income are entered in the Self-Employed Retirement section of TurboTax.