There are wide verities of brushes available in market but watercolor needs very few brushes. It can be done with medium- large round brushes for details and a flat brush for the washes.
The brush used for watercolor should be softer and denser tuft with fine tip which holds more water and pigment inside its bristle and needs less dipping in the water.
Mostly the watercolor artists use natural hairbrushes but they are much more costlier than synthetic hairbrushes. For the beginners it is convenient to use synthetic brush, as it is cheaper and easier to use which gives similar finishes.