On your 2019 tax return you enter only the amount you actually paid to the dentist in 2019. If you are making the payments directly to the dentist, you enter the total of the payments that you made in 2019. But if you financed the bill by taking a loan from a third party, and you are making payments to the lender, you enter the full amount that the lender paid to the dentist in 2019, which is probably the total bill.
The tag below your question indicates that you are using TurboTax Free Edition. You cannot claim itemized deductions in Free Edition. You would have to upgrade to Deluxe, which is not free. Before you upgrade you should make sure that you will actually benefit from claiming itemized deductions. In order to get a tax benefit, your total itemized deductions have to be more than your standard deduction. And your itemized deduction for medical expenses is not the full amount that you enter. Only medical expenses in excess of 7.5% of your AGI are actually deductible. TurboTax will subtract 7.5% of your AGI from your total medical expenses to calculate the amount of the deduction.
