If I have no clue what my original Roth contributions were how can I provide a number?

[Opus 17]

If you are under age 59-1/2, withdrawal of original contributions is not taxable but withdrawal of earnings is taxable.  It is up to you to keep those records and, if audited, the withdrawal may be taxed unless you can prove the amount of prior contributions and withdrawals and that you were only withdrawing contributions.  

Your broker issues a form 5498 each year that documents your contributions for the prior year.  You should contact your broker and get as many past year forms as they keep on file.  The IRS also has copies of your form 5498 going back 7 years you can get online.

https://www.irs.gov/individuals/get-transcript

[MaryK4]

To find your original Roth IRA contributions without records, request your historical Form 5498 (IRA Contribution Information) from your current custodian or by requesting "Wage & Income Transcripts" from the IRS. If unavailable, you can reconstruct the total by reviewing old bank statements or using online brokerage records, as contributions must be reported to the IRS. 

TurboTax asks for your original Roth contributions (known as your basis) primarily to determine if a withdrawal you made this year is taxable.  While Roth IRAs are funded with after-tax money, the IRS has strict rules on what order money leaves the account. Without knowing your original contributions, TurboTax cannot prove to the IRS that you are only withdrawing "basis" rather than taxable "earnings".