Thank you for your message.
Regarding your demand, I'm afraid that the event log sensors are not able to monitor a specific log file.
However, you can develop a script which uses the PowerShell cmdlet Get-WinEvent -LogName "" to get the entries from it, and then return the number of entries which match your filter(s).
Here is the documentation of the Get-WinEvent cmdlet: https://docs.microsoft.com/en-us/powershell/module/microsoft.powershell.diagnostics/get-winevent?view=powershell-7.1
To execute the script in PRTG, you can use the EXE/Script or EXE/Script Advanced sensor.
I invite you to have a look to the manual regarding custom sensors: https://www.paessler.com/manuals/prtg/custom_sensors