Each 1.2 volt cell has a capacity of 10 Ah. (10 000 mAh / 1000 = 10)
The cells will be wired in series (20 x 1.2 v= 24 v), and because they are in series, the total capacity of the pack will be the same as each individual cell. Therefore, you will end up with a 24volt 10 Ah battery pack.