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 at any particular time to actually focus the lens. It may be a single point manually selected by you or it may be one of a wide number of 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 the subjects behind 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, and how consistently it can do this from shot-to-shot. In some cases how quickly the system can do this is also vital.
(What follows is a rather extended technical description of the differences between dedicated PDAF sensor based AF systems found in DSLRs and image sensor based CDAF based or hybrid AF systems found in mirrorless cameras. If you don't want to read it all, please skip to the next divider just before the end of the answer)
How DSLRs focus using a dedicated Phase Detection Auto Focus (PDAF) Array and how mirrorless cameras focus using the actual imaging sensor are fundamentally different.
PDAF systems measure the difference in light from a specific point coming through the left and right edges of the camera's lens. It compares the difference, calculates the amount and direction the lens is out of focus, and sends instructions to the lens to move the appropriate amount. Then the camera takes the picture.
Mirrorless cameras don't use a dedicated PDAF sensor. Instead they use the imaging sensor itself. This has both advantages and disadvantages over the PDAF systems used in DSLRs.
- Traditional Contrast Detection AF (CDAF) systems are as accurate as is theoretically possible when properly executed. This is because the distance to the imaging sensor used to calculate AF is the same as the distance to the imaging sensor used to take the photo - after all, it is the same sensor! PDAF systems, on the other hand, must be calibrated so that the distance from the lens to the PDAF sensor is the same as the distance to the imaging sensor once the mirror moves out of the way and the focal plane shutter opens.
- Traditional CDAF system don't use phase detection to measure the degree of focus the PDAF systems do. Instead they measure the contrast of a selected point on the imaging sensor and move the focusing elements of the lens until the contrast is maximized. This tends to be a lot slower than PDAF systems.
- Because CDAF system use the main imaging sensor, this requires the sensor to be energized throughout the composition and focusing process. Energizing a sensor produces heat. The longer it is energized, the warmer it can become. Unfortunately, heat also increases the visible noise created when an image is captured. The most recent mirrorless cameras are much better at controlling the heat and the resulting noise than in the past, but it is still a consideration when shooting in high temperature environments for extended periods of time.
So traditional PDAF systems are faster and traditional CDAF systems are more accurate. But some modern mirrorless cameras have added a twist. Some DSLRs also do the same thing when focusing in Live View, which effectively turns a DSLR into a mirrorless camera. (In Live View a DSLR can and often does have just as many focus points as a mirrorless camera can.)
But camera makers have enhanced the traditional CDAF system with what is known as a hybrid focus system: The microlenses above certain pixel wells on the sensor are aimed at a slight angle to one side or the other, rather than straight ahead as would normally be the case. This reduces their overall efficiency with regard to the light striking them straight on. But when paired with another adjacent pixel well aimed at the other side of the camera, it allows them to function as part of a phase detection AF array. It takes a few dozens of pixel wells per AF point to create this array.
Even with 425 focus points (assuming these are all hybrid PDAF type points) it only takes about 20,000 pixels distributed throughout the sensor to create an array. That may sound like a lot, but when placed in the context of the 20,000,000 pixels of a 20MP camera, it is only one in 1,000 or 0.1%. The loss in light capturing efficiency is not noticeable to our eyes at all.
Hybrid AF systems usually use the PD array to get the lens very close to focus very quickly, then use contrast detection to zero in and fine tune focus. As a result the speed of imaging sensor based AF systems has pretty much caught up with PDAF based systems in DSLRs when focusing a single time on a static subject. This means the AF speed of the lens has become the limiting factor. With comparable lenses mirrorless cameras can focus as fast as DSLRs.
Where PDAF systems in DSLRs still excel is at tracking moving subjects using large, long focal length lenses. These types of lenses are not available in the correct mount for most mirrorless cameras. Using an adapter to use such lenses on mirrorless cameras tends to slow down the AF significantly (if it even functions at all) as the adapter must "translate" the signals from the camera into the "language" used by the lens. Such large, long focal length lenses also tend to have larger, heavier focusing elements that require a lot more power to move than than the typically smaller, shorter focal length lenses made in mirrorless mounts. The extra power demand of such lenses is significant when adapting them to mirrorless cameras that tend to be smaller and lighter, including their batteries which tend to have lower capacities.
When continuously tracking moving objects the top DSLRs such as the Canon 1D X (which has a 360,000-pixel RGB+IR metering sensor with 216 metering zones connected to the EOS Intelligent Subject Analysis system) not only use the focus points of the PDAF sensor array but they also incorporate the information from their multichromatic metering sensors to differentiate colors as they move across the scene.
Here's how PDAF systems function with regard to sensitivity and accuracy/consistency:
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. Sensitivity is also based on the maximum aperture of the lens being used. Such cameras 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/Consistency: 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.
The number of focus points you quoted in your question refer to PDAF type focus points on the dedicated focus arrays of DSLRs. The number of focus points quoted for a camera such as the Sony A6300 are the number of hybrid focus points on the imaging sensor. DSLRs can and do have just as many hybrid AF points on the imaging sensor. Canon calls their system the Hybrid CMOS AF III image sensor-embedded autofocus system.