Existing servers typically have 12 channels per socket, but they also have two DIMMs per channel, so you could double the number of channels per socket without taking up any more space for slots. You could also use CAMM which takes up less space.
They don't currently use more than 12 channels per socket even though they could because that's enough to not be a constraint for most common workloads, more channels increase costs, and people with workloads that need more can get systems with more sockets. Apple only uses more because they're using the same memory for the GPU and that is often constrained by memory bandwidth.
Usually this comes at a pretty sizable hit to MHz available. For example STH notes that their Zen5 ASRock Rack EPYC4000D4U goes from DDR5-5600 down to DDR5-3600 with the second slot populated, a 35% drop in throughput. https://www.servethehome.com/amd-epyc-4005-grado-is-great-an...
(It's also because of servers being ultra-cautious again. The desktops say the same thing in the manual but then don't enforce it in the BIOS and people run two sticks per channel at the full speed all over the place.)
The M1 Ultra has LPDDR5, not DDR5. And the M1 Ultra was running its memory at 6400MT/s about two and a half years before any EPYC or Xeon parts supported that speed—due in part to the fact that the memory on a M1 Ultra is soldered down. And as far as I can tell, neither Intel nor AMD has shipped a CPU socket supporting 16 channels of DRAM; they're having enough trouble with 12 channels per socket often meaning you need the full width of a 19-inch rack for DIMM slots.