> the -mt option seems to allocate core per file

this can't be true in solid archive mode, which should almost always be used (otherwise you might as well use zip). also:

  $ tee {1..10} <<< 1
  1
  $ seq 1 1000000 > 11
  $ time 7z a x.7z {1..11}; rm x.7z
  7-Zip (z) 21.07 (x64) : Copyright (c) 1999-2021 Igor Pavlov : 2021-12-26
   64-bit locale=en_US.UTF-8 Threads:12, ASM
  
  Scanning the drive:
  11 files, 78888917 bytes (76 MiB)
  
  Creating archive: x.7z
  
  Add new data to archive: 11 files, 78888917 bytes (76 MiB)
  
  Files read from disk: 11
  Archive size: 2680630 bytes (2618 KiB)
  Everything is Ok
  7z a x.7z {1..11}  19.89s user 0.09s system 205% cpu 9.733 total
  $ time 7z a -mmt=1 x.7z {1..11}; rm x.7z
  7-Zip (z) 21.07 (x64) : Copyright (c) 1999-2021 Igor Pavlov : 2021-12-26
   64-bit locale=en_US.UTF-8 Threads:12, ASM
  
  Scanning the drive:
  11 files, 78888917 bytes (76 MiB)
  
  Creating archive: x.7z
  
  Add new data to archive: 11 files, 78888917 bytes (76 MiB)
  
  Files read from disk: 11
  Archive size: 2779833 bytes (2715 KiB)
  Everything is Ok
  7z a -mmt=1 x.7z {1..11}  21.88s user 0.05s system 99% cpu 21.943 total