If you want to remove render hairs, you can connect a weight or texture map to the Density Map parameter (on the Effects page in the Hair property editor). You can determine the density of the render hairs in different areas on the hair emitter object. This parameter changes the total number of render hairs because it prevents hair from being displayed ("emitted") that would have otherwise been there.
When you connect a map, Density Map blocks out hair where the map has values lower than 1. For example, a value of 1 (such as pure white in a texture map) allows all the render hairs (as determined by the value for the Total Hairs parameter) to be displayed, while a value of 0 (such as pure black in a texture map) prevents any render hairs from being displayed. Any value in between is the probability of the render hair not being displayed.
For an example of connecting a texture map, see Connecting a Texture Map to Hair Color Parameters.
For an example of connecting a weight map, see Adding Frizz and Waves (Kinks).
