Reusable quantity

We got a second hand bundle of newborn ones, brands won't mean anything to you because we have different ones here in Aus. We had a style that used velcro instead of snaps that we quite liked. They all had inserts that snapped into the nappy which we found very easy. We had 15 in total for newborn, we only used for the first 6 weeks then she fit the regular size. We washed everyday but we wash everyday anyway so it didn't bother us. We then sold them on for the same price we paid (about $80aus) and now we have about 25 regular nappies and we wash every w or 3 days depending on how many weve gone through each day. (Obviously hot wash each day but big wash every 2 days)
The amount you need will depend on how much you intend to wash, and also factor in drying times if you live in a colder climate.

See if you can find a local nappy library that could lend you a kit to get started - that way you’ll get a sense of how many nappies you need (this is what I’ve done). I don’t think there’s a “right” answer - depends how many your baby goes through in a day and how often you want to be washing. But as a ballpark I was recommended approx 20 for full time use. Getting a kit from a library also helps to figure out which brands you like!