I'm using a gluten free Angel Food Cake mix. It asks for 12 egg whites.
Yes you can freeze them orrr..you could use them to make a lemon curd, or pastry cream to go with that cake. If you do freeze them, just decide in what quantities you will use them and portion them out into small containers or small freezer zip lock bags. They freeze very well.
Freeze individually in an ice-cube tray, then pop out and put in a freezer bag. When you thaw them, you may have to add a fresh yolk to get them to behave right.
However you decide to freeze them, be sure to whisk them first. The membrane surrounding them tends to dehydrate and toughen in the freezer, and results in clumps if you whisk them only after thawing them.