Match rates vary widely based on the quality and freshness of the original data being appended. If your list is 20 years old, the match rate might not be so good.
Check with your Database Consultant to evaluate your list before appending. They should be able to tell you the match rate and the exact number of records that can be successfully appended before proceeding.
Depending on your list, we may be able to run an append test to gauge estimated append match rates.