At any one time, there is a limit to the number of satellites of any constellation visible in the sky - example - M8 products can track two constellations concurrently. The geometries of most orbits for any constellation allow visibility of up to 14 satellites concurrently at almost anyplace on Earth -15 or 16 SVs of one constellation might be visible possible at very specific locations on the earth.
2 constellations X 15 visible satellites = 30 maximum tracking channels.
But to see the acquisition prowess of M8 receivers and to see how many channels may be in use concurrently, enable NAV-SVINFO when you start the receiver and before it has a position fix. Most likely the overwhelming data of that report will not fit in the window you have open in u-center. After you have a position fix, the receiver settles down to a smaller reasonable number of channels in use.
As clive1 states, other channels are used for a large number of reasons (and are part of u-blox proprietary algorithms hence not disclosed)