I’ve seen a whole range of strategies, and I don’t think the amount you post correlates with how successful you are. Some people post one 2K word  excerpt, other people post 30 chapters.

I posted the first 3 episodes of my book, which are around 10K words each, during the contest I participated in. I think the majority of readers read the first episode, at the very most; I’m glad I have that much posted, but I’m also glad I didn’t spam my followers’ inboxes with 30 small chunks of writing. I also really don’t expect anyone to read all of it - but it’s there if someone wants it.

The absolute most important thing, I think, is that the first thousand words you post are very strong - as cleanly edited as you can make them, strong voice, a decent introduction into the writing style people should expect to get from your book, etc. Most people won’t read more than that, and it needs to be good. It should be enough for readers to have confidence that giving you their money will result in getting something decent in return.