There are two primary types of valve extenders on the market, and these from Challenge are the "Good" type that RideCX recommends for cyclocross use for their simplicity and reliability.
Order qty "1" gets you two valve extensions, enough for a complete bike. They weigh almost nothing, include the key/tool needed for installation, and will outlast the wheels and tires you install them on. Each is 42mm long, which combined with the height of the original valve, is tall enough to be accessible even with deep carbon tubular wheels.
NOTE: to use this superior type of extender, your tubular tire (or inner tube) must have a removable presta valve core. If the core is fixed and can't be removed, this extender won't work (the vast majority of high quality tubular tires and inner tubes have removable cores).
Read on for info about the two types of valve extenders commonly offered if you are interested in the technical details: