If you make basis-only withdrawal from a Roth IRA, how is box 7 marked on the 1099R?

If you are under age 59 1/2 then "J" is the proper code.    For Tax purposes you only have one Roth IRA, but you can have as many Roth IRA accounts with various financial institutions as you want.   The Roth basis is the aggregate total of the basis in all Roth IRA accounts.   The IRA custodian has no way of knowing if you have a basis or not in other accounts so the 1099-R only reports the gross distribution and code determined by your age.   You must supply the basis in TurboTax.

You can always withdraw your own Roth contributions tax and penalty free.

Enter a 1099-R here:

Federal Taxes,
Wages & Income
I’ll choose what I work on (if that screen comes up),
Retirement Plans & Social Security,
IRA, 401(k), Pension Plan Withdrawals (1099-R).

OR Use the "Tools" menu (if online version under My Account) and then "Search Topics" for "1099-R" which will take you to the same place.

Be sure to choose which spouse the 1099-R is for if this is a joint tax return.
Be sure to pick the correct 1099-R type: Standard 1099-R, CSA-1099-R, CSF-1099-R, RRB-1099-R.

[NOTE: When you get to the "Your 1099-R Entries" screen where you can add another 1099-R, use "continue" to keep going as there are additional interview questions after that screen in most cases. You can always return as shown above.]

One of the followup questions will ask for your prior year contributions not previously withdrawn. Those contributions that still remain in the Roth will not be taxed or subject to a early withdrawal penalty. That will add a 8606 form to your tax return with the Roth contribution and tax calculation in part III.

The distribution will be on the 1040 form line 15a and the taxable amount (if any) on line 15b (11 a/b on 1040A).