I'd recommend 24-28 minimum but then even with that many you are almost washing daily especially with a newborn!! I have 50+ but i'm addicted too I wash every 2nd day.

Keep in mind those days when it's impossible to get any sun as it's raining heaps so they take longer to dry.

Try and keep a nappy stash of a mixture... hemp, bamboo and microfibre boosters for example.... pocket nappies and AIO's and fitteds. It's nice to have a mix of things.