I started a new job in November. Benefits started & they contributed to an HSA in December. Do I have to fill out the $4,150 for every month in section 2A?

The $4,150 generally refers to the high deductible health plan (HDHP) annual contribution limit for an individual Health Savings Account (HSA) for the tax year 2024

• This limit includes both your contributions and any contributions made by your employer. 
• Since your benefits and contributions started in December, you only need to report the actual amount contributed for that month.

For more details, you can refer to the IRS HSA Information & TurboTax Article HSA

"Do I have to fill out the $4,150 for every month in section 2A?"

TurboTax is responsible for populating Form 8889 based on your inputs in step-by-step mode.  Simply tell TurboTax that you had self-only HDHP coverage for the months that you were covered on the first of the month, which appears to be just December based on what you said.  For the other months, indicate no coverage.  With coverage on December 1, TurboTax will automatically determine that you are able to make a full $4,150 contribution for 2024 under the last-month rule.