This report is not included in the standard reports. There is an IVR Traffic Analysis report that may be close. It doesn't show simultaneously active, but it does show total calls for a period.
The db_cra_ccdr table shows all of the call records in the contactcalldetail table. If you could create an efficient query to count simultaneously active calls by start/end times you may be able to report on this. I would emphasize "efficient" and recommend that you profile for performance impact before implementing.
In addiion, if it's ICD, you may be able to poll RTR to get the Total Contacts in the Overall ICD report which would tell you how many contacts are active at any point in time. You could, for example, use the Get Reporting Statistics step everytime you put a call in queue to return the Total Contacts in queue, then use the Set Session Info step to put that number in a custom session variable. When the call leg ends, the CCDR will be written with that value then you could report on it based on the CCDR records.