I’m not a writer or author so I have no personal experience but I’d just recommend uploading whenever your happy with your work, people tend to upload their stories with only the prologue or sometimes with multiple chapters so their isn’t really a criteria.
If you have the whole story planned out with a clear idea of where it’s headed and a chapter to go with it then your golden.
Also uploading with the story in it’s earlier stages could mean you might receive feedback that changes the whole story itself.
I’d recommend asking these sorts of questions in the writers thread, definitely very helpful people over there.
ALSO I believe authors do beta testing right before submitting to HG.