Canon claims that EOS 6D has special capability to focus in low light, but it has only one cross-type sensor and overall only 11 focus points. Does only one cross-type focus point in the Canon EOS 6D affect its ability to focus in low light? Is it correct to assume that more cross-type focus points produce more accurate and sharper focus?
As with many things, the marketers have tried to convince us that bigger, more, or higher priced is always "better". Most of the time in photography quality trumps quantity.
Regardless of how many focus points a camera has, the camera's system primarily uses one to actually focus the lens. It may be a single point manually selected by you or it may be one of several AF points that are active based on the selected settings. If you have multiple focus points selected and more than one light up in the viewfinder, it only means that the camera is telling you that all of those focus points are at roughly the same distance.
What is more important than how many focus points the camera has is how sensitive the focus points are, and how accurately the camera/lens is able to move the elements in the lens to match the instructions from the camera.
Sensitivity: One of the ways to make AF elements more sensitive is to make them larger. The wider the distance between the two micro-lenses that split the light coming into the focus array, the more sensitive the AF system can be. The longer each line in the focus array is, the more accurate it can be. Although there is quite a bit of overlap, and in some cases shared lines, in the focus array of cameras with up to 61 focus points like the Canon EOS 1D X, all those lines start to get a little crowded on the focus array sensor. The center focus point in the 6D is rated down to EV -3. That is more sensitive than the 5DIII or the 1D X. The 5DIII and the 1D X are rated at EV -2, the 5DII was rated at EV -0.5. Sensitivity is also based on the maximum aperture of the lens being used. The 6D, as with any other camera, will be able to focus better in low light with an f/2.8 or wider lens mounted than with an f/4 or f/5.6 lens.
Accuracy: Early Auto Focus (AF) systems were "open loop" systems. The camera measured in which direction and how much out of focus the lens was, sent an instruction to move the lens a specific amount, and that was it. The emphasis was on speed rather than extreme accuracy. The most recent lenses have sensors in them that report back to the most recent cameras exactly when the lens has reached the intended point of focus. Roger Cicala, the CEO of LensRentals.com and technical guru, explains how he discovered this in a blog entry. In order for this "closed loop" system to be active, both the camera and lens needs to be one of the newer models that support this. If either one doesn't, then focus performance will be based on the older piece of equipment's design limitations. As the old saying goes, a chain is only as strong as the weakest link.
Here is a real world usage quote from Brian at The-Digital-Picture.com in his review of the Canon EOS 6D:
I can focus the 6D's center point on a subject with reasonable contrast down until autoexposure gives me a setting of 160 at 10 seconds and f/2.8 (really dark) with the Canon EF 24-70mm f/2.8 L II USM Lens mounted (note that the 6D's metering range spec is listed at EV 1-20). The Canon EOS 1D X's center point could not focus on the same subject with the same lens mounted. Note that the 6D focuses very slowly under these dismal lighting conditions - but locking slowly is far better than failing to lock.
A cross type sensor has the ability to focus on both horizontal and vertical patterns, but that in itself has little effect on the ability to focus in low light. What matters there is just how sensetive the sensors are.
Most things that you focus on have a combination of vertical and horizontal lines, so any focus sensor can handle those. Cameras generally have a few cross type sensors to handle the special situations when you focus on something that has only horizontal or only vertical lines.
A cross type sensor is of course more complicated to build and thus more expensive. By having only one or a few cross type sensors, the manufacturer can use more sensitive sensors without adding too much to the cost, so indirectly it can allow for better low light capabilities.