Demi or Demi-Glace (photo at top) is made by reducing a stock down significantly, while traditional recipes say that demi is reduced stock plus brown sauce (or Espagnole sauce), all of the commercial kitchens I’ve ever worked in, it is only very reduced stock. Traditionally reduced by half, I reduce it to about 1/4 of the original liquid. You will not get the gelatinization that you will with a vegetable stock or fish like you will with a meat stock when it’s as greatly reduced.
I typically will take a batch of demi (like the one above), cube it up into 2 inch pieces (after it’s been chilled and solidifies) and freeze the cubes slightly separated on a well wrapped sheet pan to keep out freezer odors. When frozen, I’ll put into a ziplock bag and can take cubes out as needed for sauces or soups/stews. Go forth and happy stocking!