Not sure how clear is it mentioned “on the website”, but the pinned topic on the forum says
Particularly, How Do I Level Up? | WaniKani Knowledge
The specific number of kanji you need to get to Guru is shown on your progress bar and changes with each level. For example, you need to get 17 kanji to Guru out of the total 18 kanji for Level 1. Once you hit this number and the progress bar fills up, you will level up!