We only require a minimum of 10 episodes and 5,000 words to be able to submit your novel. However, the idea behind this is that those 5,000 words are a kind of preview of a series that will hopefully be longer. I'll quote this post from a little further up in this thread:
While, theoretically, someone could submit a 5,000-word story and win one of the prizes (namely the subscriber count-based prize, as that is going to be largely decided by Tapas readers), the idea was for it to merely be a minimum requirement, not the point at which your story should end. We considered more extensive word count and episode count requirements, but we felt that would make it difficult for creators who are busy with other projects, work, and life to do. A few of us at the office have participated in National Novel Writing Month and know how hard it can be to hit a larger word count. We wanted to make it easier for more people to participate while still having a way to gauge which creators are able to follow through on finishing their stories.
Also, the advice @Croik gave is great! It's a good idea to post your story as soon as possible. You can edit any part of your novel once it's published on the site, though we recommend that you don't change the URL to your novel as that will make it more difficult for us to find it and add to the Writers Camp collection.
You can have a different thumbnail for each chapter of your novel, but they are rather small and all the ones that are not the series thumbnail will not get as much attention. And, as has been stated, you can only have one banner for your novel.
I'll post links to both of the sites that @spicy_sweets mentioned, as they're both great resources! Unsplash provides free photos that you can use attribution-free for any project. Canva provides both free images and designs. You can also add your own images (like, for example, images you find on Unsplash) and completely customize the designs.