The number of cloth pads or pairs of period underwear needed will vary from person to person, but roughly you’ll need around 6-12 with varying levels of absorbency, to match your flow from day to day. You may wish to have spares in case some are in the wash, or to carry in your bag for outings.
If you have decided that a menstrual cup or disc is for you - you only need to buy one – making them a very simple switch that can be used for years (sometimes up to 10 years if looked after well and depending on the manufacturers recommendations).