Actually, for adult fiction, 50,000 words is only a novella, most novels are 175,000 words.
If you are writing young adult, children's chapter books, or short adult fiction (novellas) than you want to aim at 30,000 to 75,000 words for a novel.
If you need some examples of book sizes verses words per story, consider this:
Short stories such as seen in magazines and compilations average at 5,000 to 13,000 words per story.
Serial length stories (in magazines) are usually 60,000 words, and published in 6 issues at 10,000 words each part or 3 issues in 20,000 words each part.
Stories like the Nancy Drew series average 50,000 to 75,000 words.
The big-big books such as Stephen King's books or many historical romance novels are 250,000 to 300,000 words per novel.
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All that aside, you should not worry about how many words to write. Just write your story, and when you get to the end, know that you have reached the end. Do not feel you have to go back and add words or cut words, just for the sake of reaching a certain word count.
After you have finished your story, count the words and figure out from there, where to market it. If it's of shorter length, aim for magazine publication. If it longer length, seek out a book publisher that does books in that legnth.
Think of it this way: word count is just another term for writer's block. Forget about word count. Finish writing. You can worry about word counts AFTER you have gotten your story written.
Good luck with you writing!