vdi infrastructure: how many vdi's are in use?

So i think the title would allready explain what i mean.

We have a vdi infrastructure, and i'm trying to find out how many vdi's are in use at any given moment.
However, i cannot find any reporting for that.
so for now the only way to see how many machines are in use is by looking at the console and look how many users are logged on there,

does anyone know of a report or tool that gives me the ability to see how many users are logged on at any given time?