There are a lot of WordPress based websites (as well as not WordPress) databases on your box. It’s necessary to get number of registered users in WordPress databases only.
Collecting WordPress databases:
mysql -Bse " use information_schema; select TABLE_SCHEMA from TABLES where table_name like 'wp_users';" > wp_users.txt
Getting the numbers:
for i in $(cat wp_users.txt ) ; do echo -n $i " "; mysql -Bse "use $i ; select count(id) from wp_users " ; echo ; done | sort -rnk2 | less