For me, the number of keywords to include is one. Experience tells me so, the algorithms confirm it, and so does user experience. I know this is a typical answer, but it's what I see working everywhere. As always in SEO, we need to stay alert and be willing to experiment — even when that goes against the most popular opinions. This is something I regularly apply to myself. Even so, in no experiment, under any circumstances, have I found a correlation between including more than one keyword and improved results.
One. A heading's job is to deliver on the promise made in the search results. "Concentrating on one single keyword phrase ensures 100% message match between the user's intent, the ad or link they clicked on, and the page they land on. Choking a heading with several keywords makes it unclear to users and search engines alike. It dilutes the page's core marketing message, indicating a lack of focus, which damages trust and conversions.