Simple answer: you don't get a deduction.
You use the term "the" charitable donation tax deduction. There is no $12K fixed, charity deduction
You can deduct your actual contributions (with limits based on income). But charitable contributions are only an itemized deduction. The total of all you itemized deductions must exceed you standard deduction ($12,550 single) before you get any benefit from a charity deduction. For 2020 and 2021, you are allowed a $300 ($600 on a 2021 joint return) deduction for charity, in addition to your standard deduction.
If you have $12k in charitable deductions, then your itemized deduction probably exceed your standard deduction and you will see some small benefit. Enter your deductions and TT will determine which is best and by how much.
