When I started out it was maybe 10-15 lbs of a few coffees as in 5 of each just to get the ball rolling. Buying in 1-2 lb increments might seem to be a valid approach, but it can take a few lbs just to get things dialed in when you're starting out. I will say don't get overwhelmed by all the possibilities with green selection and don't expect a magical batch from the start. Roasting is like most aspects of life in that you get out of it what you put into it and be patient along the way.
In the past I'd keep upwards of say 30 lbs onhand and once down to 10 or so I'd order another 20. Since COVID started affecting shipping, etc. I started keeping more and more onhand just so I always had what I wanted when I needed it. These days I've had as much as 90 lbs or so, but given that I roast around 10 lbs monthly for personal consumption it doesn't sit around very long. I rotate what I have and the longest I kept any particular green was about 11 months and the last batch was easily as good as the first. Main thing is decent storage (humidity mainly) and it will keep nicely for awhile. I buy quality from a few sellers in bulk to keep cost/shipping more affordable. My favorite way of receiving green is in heavy duty plastic bags (with holes pierced in them for it to breathe) and least favorite is when green is sent in burlap type bags. I get the reusable/sustainable part of it, but those bag fibers will end up in the coffee and need to be sifted, etc. out.