I typically go the easiest route when it comes to aquafaba (bean water). I drain the aquafaba from a can/box of beans into a measuring cup. There is usually half cup of aquafaba in each can/box of beans.
How much aquafaba per egg? I use 1/4 cup aquafaba to replace one egg.
I place it in the refrigerator if I plan to use the aquafaba the same day. However, to use it at a later date I pour the aquafaba into a popsicle tray to freeze and store in the freezer. This will last up to two months frozen. Easy peasy!