Well, it's a good idea to have one stencil brush for each color paint that you're using. If you don't have enough stencil brushes you'll have to thorougly clean your brushes before you stencil your next color which can be very time consuming.
Generally the larger the stencil opening that you have to work with the larger the stencil brush. While smaller stencil brushes work well for shading or stenciling in small areas. I've found that a 1 inch diameter stencil brush is a good general purpose stencil brush.
Buying a good quality stencil brush is a must for stenciling. A good quality brush is generally made of hog bristles or pure china bristles and has seamless aluminum ferrule. The ferrule is the metal section that holds the bristles in place. A cheap brush will shed bristles. A quality stencil brush will cost more but is well worth it. I've used cheaper stencil brushes over the years and know how frustrating it is to find stray stencil bristles in your stenciling project.