Backdoor Roth is simply a traditional IRA contribution that does not qualify to be tax deductible and is converted to Roth IRA almost immediately. Normal contribution limits apply to the after tax traditional IRA contribution ($5500 unless "catch up" applies). If you have any balance in any tIRA in your name, this gets in the way of the Roth conversion part of the backdoor Roth.
Mega-backdoor Roth is basically the same, but executed using after-tax contributions to an employer sponsored retirement account under the plan rules for that account. Not all employer sponsored retirement accounts are set up so that you can execute a mega-backdoor Roth. So you'd have to investigate your specific plan rules. The IRS limit for after-tax 401(k) contributions is the $54,000 overall limit to the account (this includes your ordinary $18,000 contributions and your employer contributions, but not your "catch up" contributions).