As far as I can see, New-EventLog
doesn't support CategoryCount.
But you can still use .NET classes directly in Powershell, so something like this should work:
$eventSourceData = new-object System.Diagnostics.EventSourceCreationData("$SourceName", "$logName")
$eventSourceData.CategoryCount = 20
$eventSourceData.CategoryResourceFile = $CategoryDllPath
$eventSourceData.MessageResourceFile = $CategoryDllPath
If (![System.Diagnostics.EventLog]::SourceExists($eventSourceData.Source))
{
[System.Diagnostics.EventLog]::CreateEventSource($eventSourceData)
}