If you are looking to have clients select a set of images for a specific purpose, such as for an album, or for further retouching, you may want to limit the number of photos they can select for a list. This can be easily done by creating a Preset Favorite List.
To create a Preset Favorite List, go to the Collection > Collection Settings > Favorite:
Creating a Preset Favorite List simply allows you to pre-define a Favorite List for a specific client (using their email address). You can enter a list name, an optional description, and set a limit to the number of photos that can be added to this list.
When clients view Favorites using the same email address, they will see the list that you have pre-created for them. They can add images to this list, and send them back to you when they are done.
The Maximum Selection you add for the Pre-Created Favorites List means that your client will only be able to add images to the List if they have not reached that limit. For example, if you have set the Max Selection to 20, they will not be able to select more than 20.