This will depend on how heavy your flow is and how often you want to wash them. There is no set amount for the number of pads you need but we would recommend starting with around 5. You can also wash your pads part way through your cycle, which will cut down on the number of pads you’ll need to get through a period.
Some pads come with inserts that you can swap rather than changing the whole pad. In this case you’ll need 1 pad per day, and about as many inserts to match the number of times you would normally change your disposable pad each day.
You can buy different sizes and absorbencies and some will come in their own washbag. To find out which size of pad you need, measure what you’re using now and choose a comparable size.