@SimonTDC - There is no hard limit to the number of slides you can have in a sidecar; however, you want to think about your audience and the best way to present your content. Will people really spend time scrolling through a really long sidecar? It's best to vary the elements you use in a story as it keeps the audience interested and engaged. Consider creating multiple sidecar that are separated by other inline sections.If you have a lot of content, think about breaking it into multiple stories and binding them together in a StoryMap collection. How you organize your story can make a big different in how people engage with it.
Most great stories that I see have 10 or fewer slides in a single sidecar, and I think you'd need a really compelling reason to have more than 20.
At this time, it is not possible to copy/paste multiple blocks of text. This is a limitation that we are not happy with, and we are working on a solution that we hope to make available in the next few months. As you mentioned, duplicating a slide and then removing what you don't need from the copy is currently the best option.
Owen Evans
Lead Product Engineer | StoryMaps