@Duncan7I am definitely in the write-while-posting group, but my buffer size is perhaps relative to the length of my story and the way I write.
Also, chapter size matters! It's very different if you're posting ~4,000-word chapters (as I am) or much longer (or much shorter) ones.
All that said, my current posting schedule is 3x/week (new chapters Monday / Wednesday / Friday). My goal is to have at least 10 chapters fully (*) edited and ready to publish at all times. More is nice, but see below.
(*) I always reread chapters just before publishing (e.g. the weekend before posting). There are almost always changes in that last pass. I've found I see errors I've missed otherwise and I also tend to see style issues I've skipped over when it's a last-minute reread. Most likely, this is because another chunk of time has passed and I'm looking at the chapter with relatively new eyes.
My other goal is to have at least 40 chapters written beyond the current publishing number. That allows me to update as necessary (which happens fairly often). Such changes are often adding story elements I missed (I tend to write the major events, then realize I missed subplots A, B, C, and D and need to at least reference them). It can be anything, though. If I've edited too many chapters, that means those changes need to go out to all of my editors 'out of cycle'. I generally try to avoid that when possible, but sometimes it's necessary.
This is for books with between 89 (current low mark) and 159 (current high mark) chapters. So, about 1/2 to 1/4 of the book (minimum) written beyond what is currently being published.
I can still adjust based on reader feedback, but sometimes it's a slow process. That is often good, I think. Fast updates are available if needed, but there's much less temptation to make major changes to accommodate some reader point (however valid it may be).