We created the Artwork > sketchbook category a few years ago for people to collect their sketches in one single thread, like a in a sketchbook, also making it easier for others to see all the sketches in one place. Unfortunately many people didn’t really understand or bothered with this rule and kept posting every single sketch in its own topic.
We now have added a plugin to Discourse that allows us to limit how many topics a user is allowed to post in Artwork > sketchbook and limited it to 5 for now (let’s see how it goes). So you still can have multiple topics if you want to group sketches by theme or style, if you want or just post into a single one. But you can’t create more topics if you reached the limit. This of course only affects Artwork > sketchbook and none of the other categories.
If you happen to be one of the users who have more than five topics in the category already, you can ask us moderators to help you merge them, or you have to delete some of your topics and post their content into one of your existing topics.
If you encounter any other issues with the category or the topic limit, you can report it in this thread.