In very general terms ...
It takes a minimum of 3 satellites to determine your latitude/longitude POSITION (but not elevation)
It takes a minimum of 4 satellites to determine your ELEVATION, but typically with +/- 50 ft. accuracy, or worse.
Of these minimum 4 satellites for POSITION and ELEVATION, ideally, one would need to be nearly over your head and the other three near the horizon and 120 degrees apart for best accuracy (i.e., one north, one SE, and one SW). This seldom happens, and if it does, it doesn't stay that way for long. Therefore, having a few more satellites available allows the GPS "firmware" to select the satellites with the best distribution in the sky to determine the most accurate position and elevation. It's not really the number of satellites above your head, or those with blasting signal strength, but their more-or-less equal distribution in the sky that is important.
DJI has determined 6 satellites is the minimum number to find the best "fit" based on distribution. For example, if several satellites are clustered relatively close to each other, say to the east, the firmware may discard a couple of them for being too close to each other and will attempt to use the best 4. And, of course, if one of those 6 satellites drops below the horizon or gets obstructed by a building, you no longer have 6 satellites for the DJI firmware to use and it can quickly become "lost."
GPS uses a mathematical model of the Earth's surface, but we all know the Earth is not perfectly round and not all ocean sea levels are zero feet ASL (otherwise, you wouldn't need locks in the Panama Canal). This is why calibration is so important ... you're telling the DJI GPS exactly your starting elevation.
As a general rule, elevation error is about 1.5 times worse than your position error. If your DJI returns home RTH and lands 15 feet away from the start, generally no biggie. But that same error in elevation (15ft x 1.5 = 22 feet) could make for a hard landing, thinking the ground is still 22 feet away, or (gasp) 22 feet in the air.
If you take off with only the minimum 6 satellites, you are pushing fate that one won't drop below the horizon or become obstructed during your flight, or 2-3 are closely clustered together in the sky for poor elevation derivation, or there are none above your head. Therefore, strive for as many satellites as you can get, like at least 8. If you take off with only the minimum 6, at least keep it LOS for a manual recovery. For those of you achieving 10+ satellites, you are almost guaranteed a good distribution of birds.
Use one of the GPS calculators often cited on the forum, or bring along a handheld GPS unit that shows you where the satellites are. The moral of the story: see where the satellites are for at least 6 being fairly well distributed throughout the sky. Distribution is more important than just the signal strength. Avoid flying if most of the satellites are clustered together in one part of the sky or too close to the horizon. Usually, you only have to wait 10-15 minutes for the constellation to change to a more favorable condition. That 10 minute wait might ensure your DJI returns properly.
(We use GPS at my work for determining ionospheric dispersion based on GPS timing and lock signals).
Hope this helps explain why DJI requires at least 6 satellites, why even that can often cause problems, and why striving for more satellites with good distribution is really the key over signal strength, especially for RTH.
Good luck to all.
Paul