bbpress just uses wordpress login, so you can display the number of wordpress users online
eg
WP-UserOnline
http://www.wpbeginner.com/plugins/how-to-show-real-time-online-users-in-wordpress/
but as to how many are in a forum, I don’t know of anything that would be clever enough to do that
Hi,
My GD bbPress Toolbox Pro plugin has users online status tracking feature, enhanced statistics block for forum front end, enhanced statistics widget (and a lot of other features):
https://plugins.dev4press.com/gd-bbpress-toolbox/
If you want to test plugin (admin side and front end), you can request a demo here:
https://www.dev4press.com/request-demo/
Regards,
Milan Petrovic