The Digital Downloads add-on allows users to download images from a gallery, either individual images or an entire gallery. It also allows you to enable images and galleries to be downloaded for free in instances where downloads may have been a part of the session fee. With the release of Digital Downloads 2.4, it is now possible to set a limit on the number of free downloads a user can make which is tracked based on the user's IP.
You can set a global option for the maximum number of downloads amongst all galleries. You can also set this on a per gallery basis as well. If set on a per gallery basis, this overrides the global number.