I know where to find the list of users, but where does it say the total number of users on Super User/Stack Overflow/Server Fault?
Go to the users page, then go to the end and see how many pages there are. Multiply the number of users per page by the number of pages minus 1, then add the users on the last page.
users = (#perpage * #pages-1) + #onlast page
For some reason a large portion of users are not listed in the users page and this leads the method of calculating based on the user listing to be inaccurate. Using the described formula one gets around 103K users.
However using one of these two alternate methods:
There is about a 20% difference between 103K and 130K. I believe this question is a duplicate of the Where can one find the total number of unique users in the entire Stack Exchange network? which is where the two more accurate methods are described.