You don't have to start all in with cloth pad. You can always start substituting with something that suits your budget. Or start with something that will suits your comfort.
But if you are ready to get the full set, here's some helpful tips for choosing the right amount.
Now, consider your own cycle. Which style pad do you prefer and when? How often do you find yourself changing those disposable pad? Are some days heavier than others, so you swap in a fresh pad more often?
Consider absorbency and coverage. For instance Dahlia (Regular Pad) absorb up to 5 hours and Violet (Overnight Pad) absorb up to 6 hours. Iris is perfect for wear at the end of your cycle or even for everyday use.
Basically the amount of absorption are similar to the disposable pads. So you can already determine the total pieces you will use.
Tip: If you have a long cycle or heavy flow, you can "double" the size of your cloth pad stash by doing a load of laundry mid-way through your period.