Number of users via web hook or S2S call
Hmm, not really...
You can get the number of sessions created in an hour with this call: https://getbraincloud.com/apidocs/apiref/#capi-globalapp-sysgetdailycounts
But brainCloud doesn't otherwise track a concurrent session count per app.
There are certainly 3rd party analytics packages that you could link into your client that would give you that sort of information though.