Up to what % of charitable donations can I claim?

A charitable donation is only entered as an itemized deduction on Schedule A.  You can enter the entire donation on Schedule A.  However -

 

The total of all your itemized deductions on Schedule A must be greater than the standard deduction for your filing status to have any tax benefit.

 

Standard deductions for 2019

• Single - $12,200 add $1,650 if age 65 or older
• Married Filing Separately - $12,200 add $1,300 if age 65 or older
• Married Filing Jointly - $24,400 add $1,300 for each spouse age 65 or older
• Head of Household - $18,350 add $1,650 if age 65 or older

 

For 2020 (Covid law), you are allowed a (up to) $300 "above the line" charitable deduction.  Even people using the standard deduction can claim it. 

You list the entire amount of the charitable donation. Whether you get any tax benefit from it depends on whether the total of all of your itemized deductions is more than your standard deduction.  For example, if you paid $5000 in state income tax and $5000 in mortgage interest and those were your only other itemized deductions, then the total of your itemized deductions with the charity donation would be $25,000. That would make your itemized deduction $200 larger than the standard deduction.