You typically need one or two ascenders, depending on the climbing style and the gear you are using. If you are using a single rope and a basic belay device, then you will only need one rock or tree climbing ascender. However, if you are using a double rope system or an advanced belay device, then you will need two ascenders. In addition, some climbers prefer to use two ascenders even when ascender climbing with a single rope, as it provides an extra measure of safety. Ultimately, the number of climbing ascenders and descenders you need will depend on your personal preferences and the type of climb you are undertaking.