The sum of boxes 21 and 23 cannot exceed 12 on your T4A(P) entry.

How do you answer the questions when a person has a permanent disability (Qualifies for disability deduction) and retired decades ago? Disability Box 21 has to be 12mo. So I leave Box 23 blank?  

The Disability Tax Credit is a CRA tax credit (not a deduction). It is not directly related to CPP Disability payments.
If you are receiving CPP Retirement benefits for 12 months in 2018, you should report these from box 14 of your T4A(P) and enter 12 from box 23.
You should not enter anything in box 21,

Your DTC amount is entered elsewhere on your return. It never appears anywhere on a T4A((P) because it is nothing to do with CPP.