That really depends on what kind of writer you are or how you're gonna be posting it.
Like, if you're trying to get immediate notice or build a Patreon or something, I suggest getting at least around 20 chapters, the first ten you release very fast, like, two every day for almost a week.
Then you put some as advanced chapters on Patreon and keep it like that, behind different levels of support, like, at least three.
If your goal is to get money, make sure to use market tricks like making the middle option less attractive than the more expensive one.
I mean, yeah, it doesn't feel very good, but you're trying to make a living and, supposedly, you're giving people content for free, they'll only support you to get that content earlier, or just because they like your story and want to help you out.
In the end, if you manage to make a living of it, they'll actually benefit even more since you'll have more time and motivation to write.
Money isn't the only reason you may want to have a reserve. Having a surplus of chapter also allows you to make corrections based on feedback, like, if a reader points out something and you think it's good, you can change your advanced chapters more easily if they aren't posted.
On the other hand, if you're a slow writer, have trouble finding the motivation or you don't have a set schedule, it may not be in your best interest to even have a surplus of chapters, posting them fast may actually incentive you to write more.
If you're posting at a set schedule and, suddenly, your reserve runs out or you start slowing down a lot, you may actually annoy your readers a lot. I mean, a lot of people will understand, but some don't, and it's the extremes that tend to be more vocal.
Here in SH, I find that readers are mostly very understanding, but the vast majority won't say anything about it, also, a few bad apples are always around, and getting insulted by the more vocal minority can feel really bad.
Also, I find that anything between 1.5 and 3k words is a good number to aim for at the beginning. Long chapters can be hard to do if you don't have a lot of experience and very small ones can leave a reader unfulfilled. But it really depends on your story and skill.
I have read a story where the author only posted 500 to 600 words every chapter, but he was so skilled it was great, although it was more comedy than anything, I don't think it would work as well in other genres.
I have also read 20k words chapters that keeps my interest the entire time, but those are VERY rare.