The MB will show how many active connections there are (ie actively transmitting data, not idling). That screen can take 30 seconds or so to refresh though, when you first go to it it will show 0 connected.
There's no way to see how many are connected to the airave.
I think the airave has a limit of 16 concurrent connections and the MB is 64? But lilotimz would know for sure.
As for MVNOs, I'm not sure what PLMNs the MB and Airave broadcast, if any, in addition to 310120.
Is there a way to tell how many people are connected at any one time to a magic box? Do the sprint owned MVNOs have access to things like magic boxes, airraves and sc1000s? Looking at adding sprint equipment to my restaurant...