accept number of files html : I use these codes for creating a form which accept one or more files. not zero files.

        <!DOCTYPE html>
    <html>
      <head>
        <meta charset="utf-8">
        <title>Problem</title>
      </head>
      <body>
        <!-- Insert form here -->
        <form id="uploader">
        <input type="file">
        <button type="submit"></button>
        </form>
        
      </body>
    </html>

but it's not working. How can i do that?
thanks a lot