Feature summary (what you would like to be able to do and where):
When on the 'Get IPs' mode for a user, a link to check the /64 has been added. This was done as in most cases a /64 is assigned to one person. Being able to see the number of other users using the /64 would help to determine whether there is a need to run a check on the IPv6 address.

Use case(s) (list the steps that you performed to discover that problem, and describe the actual underlying problem which you want to solve. Do not describe only a solution):
As a CheckUser looking at the results in the 'Get IPs' mode for a user, I want to know whether other users have used the /64 range that a check link appears for. This is so that I can know whether running a check on that range would produce results for IPs or other accounts, and thus whether running the check is worthwhile.

Benefits (why should this be implemented?):
A CheckUser currently has no way to know if the /64 range is used by other users. Because IPv6 addresses can be very dynamic inside the /64, I will run a check on that /64 in all but a few cases so that I don't miss results. Being able to see in advance that the check would not help me find other accounts would reduce the clicks I need to carry out a check.

Extra info
I would see this being implemented by adding to the existing "~X from all users" such that it would be "~X from all users and ~Y from all users on /64" with or without the X from all users part. Another method could be used, but having it close to where this information is already shown is useful.