The maximum pax capacity for most rental agencies is 9 or so. Hertz rents a Ford Transit, Sixt a Renault Trafic, and Avis a Fiar Ducato etc. You'll probably need to replan for two vehicles or rent a small bus with a driver. there are many companies that will rent a 19 pax bus with driver, but seemingly none without. search fro "bus and pullman Italy" and you'll find a number of them.