This category is meant to have all your sketches in one singe topic (like your sketch book) but you can have up to 5 topics in this category, if you want to group them in a way (for example by theme or style).
If you find yourself in a situation that you can’t post any more topics because you previously posted every sketch in its own topic, you can ask a moderator if they can merge topics for you or you have to delete some of your topics in this category.