A newborn would need 12+ cloth diapers a day. I recommend stocking up on enough to have at least 2 days worth of cloth diapers so while the current days diapers are in the wash you'll have plenty of cloth diapers to get you through the next day.
So 24 diapers should be enough but you can never have too many. Even if you buy more than you actually will use it's still cheaper than all the disposables that you'll be purchasing every month.
Plus cloth diapers are a lot better for babies skin.