The classic ratio of stock to glace is 16:1. This means that 16 cups of chicken stock (or 1 gallon of chicken stock) will give you 1 cup of glace, when you reduce it down for hours. The flavors and collagen become even more concentrated as the chicken stock reduces down into glace.
You don’t have the reduce the stock down all the way to one cup. For example, if you reduce a gallon of stock down by half, you have demi-glace, which is amazing to have on-hand in your fridge or freezer. True glace just takes the process a step further, creating a product that is incredibly concentrated and almost pure collagen.
Some people will fill an ice cube tray up with glace, creating little frozen cubes that can be transferred to freezer bags and stored in the freezer. You can keep these safely stored for months, pulling out a cube or two for whenever you need to use it.