There’s no real rule here and (as with much of SEO) this is down to research, as Google rarely share what they’re looking for to rank your site. Our advice is this: Don’t think about how many keywords, think about nailing a subject with the best content you can create. It’s not about ‘stuffing’ keywords on a page; it’s about picking a topic that your readers find compelling, and going into detail. Create articles full of value; not full of keywords. You’ll naturally create some ‘keyword-rich’ copy when you do this!