I noticed our Cronlogs always said "0 out of 4 archivers running currently" - regardless of the actual number of running archivers.

This was, because our cron for the core:archive job had the matomo-domain parameter in front of the core:archive command:
console --matomo-domain=my.matomodomain.com -vvv core:archive >> cron_daily.log
but CronArchive.php checks for strpos($process, 'console core:archive') !== false in hasReachedMaxConcurrentArchivers().

Changing the check to strpos($process, 'core:archive') !== false reports the correct number of running archivers.