Just a quick note. If you are looking for all private channels created in Teams, you can use the following line of PowerShell:
Get-SPOSite -Limit All -Template "TEAMCHANNEL#0"
If you are just looking for the amount of private channels (to see if it is actively growing), use:
(Get-SPOSite -Limit All -Template "TEAMCHANNEL#0").Count
I also wrote a PowerShell module to let you easily & securely connect to any Office 365 tenant. You can find more information here: https://sharepointrelated.com/2018/06/12/powershell-easy-secure-office365/