MariaDB - So many threads?
Once you have had some activity, I think the number of threads varies between

(misc) + thread_cache_size
(misc) + max_connections


thread_concurrency was specific to Solaris 8, and has been removed from newer versions of MySQL/MariaDB.

Where (misc) is 1 for the main process plus replication slave threads plus innodb read/write/purge threads.  (Thanks danblack)