Webtoon doesn't make contests every year, not to mention that the requirements for each one is different, therefore, quantity of panels, genre, focus, theme, and other aspects will vary depending on the kind of contest they'll host, usually, 3 to 4 months prior to the launching of the Contest itself.
Call to Action was this year's. 2021 didn't have a contest because of the launch of Canvas de Webtoon (The staff in charge of webtoon english, who are from Los Angeles, are the same staff for the spanish platform, there was a contest in the Spanish platform). In 2020 was the Short Story Contest
Of coursel there are other contests depending on the language, like Webtoon Indonesia and Webtoon France had other ones this year, but these are no guarantee that each year there'll be a challenge. You can participate on their events in order to attempt to promote your story tho