If you go to the Upload field properties make sure you have enabled the the option "Allow Multiple File uploads" and also specify the number of files a user can upload at once. The default seems to be 5, but you can make it 10, 20 or whatever.
Be aware your hosting service may place limits on what you can or can't upload - and that's outside the Machform software's control. If you try out the demo, you can see this for yourself https://urrfbcicadz.machform-trial.com/index.php, or else double-check you have enabled the above-mentioned options. I just suspect you have inadvertently overlooked them.