I can tell you what I know. I've been marginally successful ($1300 in one month was my best, with about $375 for one article). Reads and claps are important because that is how Medium bases its pay. More reads by members = more money. I gathered a list of publications by noting which ones other people publish to, and then I follow them. AND THEN I apply to be a writer on them. The list of pubs changes, so I don't know if there is a central place to find them all. There are A LOT. Getting paid requires you to join the Medium Partner Program and to set up a Stripe account. It's not hard. As for how much to write, write as much as you can do well. I have seen a lot of articles there the author is obviously going for quantity instead of quality. If I write 3 or 4 in a week, I've had a stellar week. I hope this helps!