Authors, especially new ones, struggle with how many words to write. There are two rules to follow. The first is to keep close to industry standards if you want to attract a publisher. The second rule is that there are no rules.
These days, with so many publishing options, you don’t need to follow industry norms. You can write the exact number of words that best tells your story.
However, if you do want to attract a traditional publisher, or if you want people to more easily accept this book by an unknown author, it helps to follow the guidelines in this infographic. The guidelines run from board books through young adult fiction all the way through epic fantasy. Some genres have more rigid guidelines, others are more relaxed.
Remember that these really are guidelines, not rules. The most important thing is to tell a compelling story that gets good reviews and readers hungry for your next novel.
Infographic Source: https://thgmwriters.com/blog/average-book-word-count/