re: "Do I have to let the 17 people I didn't hire know that I didn't choose them for the project?"
Don't try to contact all of the other candidates individually.
If you close the job post, they will be notified.
Also, I believe that when you HIRE one individual, there is a checkbox that lets you specify that you want the other candidates to be notified with a reason they were not hired, such as "hired somebody else."
There is no requirement that you contact unsuccessful candidates. Best practice is to let applicants know why they weren't chosen so that they can improve, and will think kindly of you should you post a job in the future. It's something of a pain to do it, but if you plan to hire more than once or twice it's worth it.
I acknowledge every response and keep applicants apprised of selection status. I tell them why they didn't win because I want them to get better. In this community, making one freelancer better helps everyboy.
I want freelancers to have a good feeling about me and my company. I've hired half a dozen freelancers whom I had rejected for an earlier job.