If you make the nondeductible contribution and a full conversion in the same year then your basis on line 14 should be $0. The only exception would be if had a loss before you converted the funds then it is possible to have a basis on line 14.
If you made the contribution for the previous year and the convert it in the current year then you would have a basis on line 14 on the previous Form 8606 that would be carried over to line 2 of the current Form 8606.
Note, this assumes your traditional IRAs were empty before starting the backdoor Roth procedure.
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