Free users have a download limit of 100 downloads per track, Pro users have 1,000, and Pro Unlimited users have unlimited downloads available for their tracks. You can upgrade to Pro or Pro Unlimited here.
Enabling downloads means that your listener can have a copy of your original file format that you have uploaded to SoundCloud.
You can enable, or disable downloads for your listeners through your track's Permissions tab.
You can enable or disable downloads for multiple tracks at once through your Tracks page.
Click on the tracks you would like to change the download settings for, and click ‘Edit tracks’ and choose ‘Permissions’ from the dropdown: