This would be one for powershell, query the Get-WMIObject Win32_Process cmdlet see how many sqlserver.exe processes are running and if > 5 send you a mail.

Could then set it up as a scheduled task using windows schedule, or as a Agent job on each instance, but I would go with scheduled task as that way if a node fails you dont get between 5 - 10 emails saying the same thing.

Edit

This is a starting block

Get-WmiObject win32_process | where-Object {$_.ProcessName -eq "sqlservr.exe"} | select name

Will get the number of processes running, would just need to aggregate them and find a send mail function of the web, then build in the logic.