CMC stands for carboxymethyl cellulose. It's a fine powder that makes the sugar paste thicker/firmer (the formed parts hold the shape better) and speeds-up the drying time. Other names for CMC are tylose powder and tylo powder.
CMC is NOT the same thing as gum tragacanth. CMC is a synthetic substitute for gum tragacanth. CMC works quicker than gum tragacanth, so you don't have to wait many hours for the paste to get thicker/firmer.
The amount you need to use depends on:
The type and brand of paste you are using (the softer the paste, the more you need to add) .
The part you want to make:
parts that support the weight of other parts should have more CMC
parts that don't support any weight don't need that much CMC.
The amount of modelling involved:
more complex shapes require more time to model, so the paste should be a little softer
simple shapes f.ex. a flattened oval, don't require a lot of time to model, so you can add more CMC.
some cake decorators prefer to work with softer paste and let it dry for longer
some cake decorators prefer thicker paste that dries out faster.