Kindle Serials are stories published in episodes. When you buy a Kindle Serial, you will receive all existing episodes on your Kindle immediately, followed by future episodes as they are published.
Wait for it.
This is from the submission guidelines:
A minimum of two episodes in a Word or text document. Each episode should be a minimum of approximately 10,000 words. We’re open to considering projects with more or fewer words per episode, where it makes sense. The complete book doesn’t need to be already written.
Boldfaced emphasis added by me.