As long as you're running on a version which supports the logging of IPv6 addresses (v7.6+) you can check the database for a snapshot of how many users have logged in from unique IPv6 addresses with the following queries.

You can get a snapshot of how many posts are made by unique IPv6 users by running:
SQL Query
SELECT COUNT(DISTINCT `POST_POSTER_IP`) FROM `ubbt_POSTS` WHERE `POST_POSTER_IP` LIKE '%:%'

Likewise you can see who has registered from an IPv6 address with:
SQL Query
SELECT COUNT(DISTINCT `USER_REGISTRATION_IP`) FROM `ubbt_USERS` WHERE `USER_REGISTRATION_IP` LIKE '%:%'

Or check the user User Last Login IP:
SQL Query
SELECT COUNT(DISTINCT `USER_LAST_IP`) FROM `ubbt_USER_DATA` WHERE `USER_LAST_IP` LIKE '%:%'

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