Hi Checco,
If you want to get the 'Causale' nodes then above Shiva given XPath is correct. If you want to 'TurnoSerale' nodes then use the given below XPath:
count(../my:Persone/my:TurnoSerale[@my:Causale = "Turno"])
By using this XPath you can get the 'TurnoSerale'' nodes with the 'Causale' node value is equal to 'Turno'.