Search notes:

apt update

Getting some statistics before updating: 
$ apt-cache stats
Total package names: 100337 (2,809 k)
Total package structures: 89372 (3,932 k)
  Normal packages: 59590
  Pure virtual packages: 924
  Single virtual packages: 19360
  Mixed virtual packages: 379
  Missing: 9119
Total distinct versions: 63056 (5,549 k)
Total distinct descriptions: 123265 (2,958 k)
Total dependencies: 386354/105572 (9,416 k)
Total ver/file relations: 64664 (1,552 k)
Total Desc/File relations: 57729 (1,385 k)
Total Provides mappings: 23904 (574 k)
Total globbed strings: 192227 (4,345 k)
Total slack space: 109 k
Total space accounted for: 34.2 M
Total buckets in PkgHashTable: 196613
  Unused: 124934
  Used: 71679
  Utilization: 36.4569%
  Average entries: 1.24684
  Longest: 16
  Shortest: 1
Total buckets in GrpHashTable: 196613
  Unused: 118051
  Used: 78562
  Utilization: 39.9577%
  Average entries: 1.27717
  Longest: 6
  Shortest: 1
Count the number of package before updating. Note: 59969 is equal to the number of normal packages (59590) plus the number of mixed virtual packages (379): 
$ apt-cache pkgnames | wc -l
59969
Updating, first attempt: 
$ apt update
Reading package lists... Done
E: Could not open lock file /var/lib/apt/lists/lock - open (13: Permission denied)
E: Unable to lock directory /var/lib/apt/lists
W: Problem unlinking the file /var/cache/apt/pkgcache.bin - RemoveCaches (13: Permission denied)
W: Problem unlinking the file /var/cache/apt/srcpkgcache.bin - RemoveCaches (13: Permission denied)
Updating needs admin privileges… 
$ sudo apt update
Get:1 http://ftp.debian.org/debian bullseye-backports InRelease [49.0 kB]
Get:2 http://security.debian.org/debian-security bullseye-security InRelease [48.4 kB]
Hit:3 http://deb.debian.org/debian bullseye InRelease
…
Get:20 http://deb.debian.org/debian bullseye-updates/main amd64 Contents (deb) [68.7 kB]
Fetched 50.1 MB in 16s (3,080 kB/s)
Reading package lists... Done
Building dependency tree... Done
Reading state information... Done
36 packages can be upgraded. Run 'apt list --upgradable' to see them.
Check stats again. Some numbers are slightly different: 
Total package names: 100339 (2,809 k)
Total package structures: 89374 (3,932 k)
  Normal packages: 59592
  Pure virtual packages: 924
  Single virtual packages: 19360
  Mixed virtual packages: 379
  Missing: 9119
Total distinct versions: 63070 (5,550 k)
Total distinct descriptions: 123279 (2,959 k)
Total dependencies: 386470/105446 (9,417 k)
Total ver/file relations: 64674 (1,552 k)
Total Desc/File relations: 57743 (1,386 k)
Total Provides mappings: 23906 (574 k)
Total globbed strings: 192116 (4,344 k)
Total slack space: 105 k
Total space accounted for: 34.2 M
Total buckets in PkgHashTable: 196613
  Unused: 124933
  Used: 71680
  Utilization: 36.4574%
  Average entries: 1.24685
  Longest: 16
  Shortest: 1
Total buckets in GrpHashTable: 196613
  Unused: 118050
  Used: 78563
  Utilization: 39.9582%
  Average entries: 1.27718
  Longest: 6
  Shortest: 1
count the number of packages again: 
$ apt-cache pkgnames | wc -l
59971

See also

apt
apt-get update

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