So basically there are three different types of roux. They are White, Brown and Blond. A roux for those who don't know is a cooked mixture of equal amounts by weight of flour and butter that is used to thicken sauces. The difference is how long you cook them, usually for a white roux you only cook it for a minute or so. A Blond roux is cooked for 3-4 mins and a Brown roux is cooked until you have the color you are after. Be careful not to over cook the roux. It is easy to go from a brown roux to a black one.
The best way to know how much roux to make is to use 2oz of flour and 2oz of butter per 1quart of liquid.
Now to talk about Demi Glace a little bit. This is made by half brown stock and half brown sauce reduced by half. This is generally used to make sauces like Chasseur sauce. It is common now days for people to not make the Demi Glace like they used to. A Modern Demi is made from just reduced brown stock until it is thickened. This is still a bit runny and not nearly as sauce like.
One thing to remember is a Demi Galce is not like a Glace De Viande. A Glace De Viande is just for flavor and doesn't make sauces.