SoWell, that turned out to be really simple. Check out a simple RowCount:

transform CountRows:
	Context.Items.RowCount = 0 unless Context.Items.RowCount
	Context.Items.RowCount+=1
	RemoveRow()

	OnComplete:
		SendRow( Row(RowCount: Context.Items.RowCount) )

And then we have a more complex one, summing two columns:

transform CalcSumOfSalaryAndId:
	unless Context.Items.IdSum:
		Context.Items.IdSum = 0 
		Context.Items.SalarySum = 0
		
	Context.Items.IdSum+=Row.Id
	Context.Items.SalarySum+=Row.Salary
	RemoveRow()
	
	OnComplete:
		SendRow( Row(
			IdSum: Context.Items.IdSum, 
			SalarySum: Context.Items.SalarySum
			) )

 So, basically we have an initialization section, processing, and when all the processing is done, you can send new rows downward in the pipeline.