Each individual is different. A good gauge of how many cloth pads you need would depend on how many disposables you use. The number of hannahpad cloth pads you would need should be the same as with disposables or even less.
Most women would change out their disposable pads frequently as they become uncomfortable or bad odour, even if their pads are not full. With hannahpad, there is no odour and it is just as comfortable and breathable as wearing your underwear, hence you may not need to change our your hannahpad as often. However, every individual is different, your period flow and comfort level will also differ.
We recommend that you try at least 2-3 different sizes and add on more later when you have a better idea of your needs.