The Music Key Signature FLAT table works in same manner as the table of sharp keys: The column on the left names the scales/keys and the right hand column indicates the name and number of flats in each.
For example, if you want to know how many flats in the key of Ab major go down the left hand column to Ab and go straight across to Db in the right column. The key of Ab has Db and all of the flats above it. so, the key signature for Ab major is 4 flats: Bb, Eb, Ab, and Db .
It is important to memorize this table. practice naming the flats in all of the keys. It would also be good to practice writing out each scale using your knowledge of this table.