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Google has reiterated that when it comes to being eligible to show course rich results in Google Search, you need to have marked up at least three courses on your site. The guidelines were updated to say "you must mark up at least three courses," but this has always been a rule.
The revised documentation were updated specifically on this section of this page and it says "You must mark up at least three courses. The courses can be on separate detail pages, or in an all-in-one page. You must add Carousel markup to either a summary page or an all-in-one page." Those two sentences are new to that page, but it has been on the carousel documentation for a while now - so technically, the guideline is not new.
Here is what course rich results may look like in search:
Forum discussion at Twitter.
