It is difficult to provide an accurate number of active users. The main reason for this is that BoS features a cross-cohort user base. This means that, users from several HSC cohorts have previously used, and in many cases continue to use BoS after finishing their HSC. There are multiple examples of users who are still active on BoS, including:
Users in the moderating team as well as premium members (many of whom were also in the moderating team at some point in time). Such users remain active on BoS, despite having joined many years ago.
Regular users who happened to be quite active on BoS in the years leading up to their HSC. Some of those users may also find themselves using BoS from time to time to make occasional contributions and/or for nostalgic purposes.
Regular users who have completed their HSC and remain active beyond year 12 to contribute to the BoS community. Some users may be grateful for the assistance that they may have received, such as through messages in response to one or more of their threads, and may therefore wish to give back to the community that helped them during their HSC journey.
While I cannot guarantee the accuracy of this range, I would be inclined to believe that the number of active users on BoS (this does not suggest that all of those users are online at the same time) could be anywhere between hundreds and thousands of users.