Is there anyway to limiting the users for a particular HTML Dashboard in Splunk?
you can limit the number of concurrent searches by, modifying limits.conf.
but for limiting the number of users concurrently, I don't think there is any implementation as such from Splunk side.
You can do a limit on your server itself. That is limiting the number of connections, but that's a long shot.
F5 LTM/APM coupled with iRule to limit concurrent sessions.