what is a retirement claim number?

Who or where is it asking for it?  If you get retirement pension or IRA distributions you get a 1099R to enter into your tax return.  

A retirement claim number is an alphanumeric identifier used for retirement accounts and benefits, often referred to as a CSA (Civil Service Active) or CSF (Civil Service Final) number

You can find your retirement claim number on several official documents:

1. Welcome letter: The claim number is in the top-right area of the letter, which is sent to annuitants shortly after receiving the retirement package from their former employing agency.
2. Retirement card: The claim number is on the front of the card, included with the welcome letter.
3. Annuity payment statements: The claim number is in the top-right area of the statement.
4. 1099-R tax form: The claim number is in the top-center area of the form.
5. Correspondence letters: The claim number may be located in the top-left or top-right area of the letter, or in the body of the letter. Note that claim numbers are not always included in correspondence letters to annuitants

For more detailed information, you can refer to the OPM's official guide