I used a service up until nine months, so similar starting point to you (and quite economical, since she's just into med sizes, and is growing much more slowly).
I'd assume you'll change him every couple of hours on average, and most people seem to wash every 2 or 3 days - so 20 or so should be enough. size will depend on the brands/styles. in some it's important to look at measurements as well as weight.
Assuming you already know the different types and their pros and cons, I'd have a good hard think about how much things like drying time, convenience, the ability to go coverless and being able to get everything very clean matter to you.
I'd get a little bit of everything to start with, so you can try it all out and see what you like.
My preference is for things that are quick to dry and easy to clean, so I rule out AIOs. I like being able to go coverless, so I like the idea of a fitted, but the slow drying, etc makes me prefer a non-waterproof pocket.
If you were me, I'd get a dozen prefolds (and trim them), incorporate some receiving blankets into my stash as flats (to try), one prorap cover (velcro), one bummis super whisper wrap cover (either snap or velcro) and one imse vimse night cover (velcro), one fuzzibunz (snap), one happy heiny (velcro), one swaddlebees (either snap or velcro) - inserts (some hemp and some microfibre for those three pockets), and one or two fitteds.
That way, you can try them all out, while jumping straight into full time cloth but with stash to build. If you don't like the prefolds you can just have them as backup - $24 is a small investment, or you can make them into fitteds, or cut them down and use as stuffing for pockets or sell them. You also get to try velcro and snap closures, and different fabrics. I thought I'd love microfibre, but I love hemp instead! The pockets I thought I'd love, I don't, and the ones I thought I'd just try - I love. And you can bet it'll all change as she grows.