Falessi, Pafi, "Veicoli da comattimento dell'Esercito italiano dal 1939 al 1945", 1976 report 75 rounds for the 75/34 gun and 600 rouds for the 8 mm machineguns.
Benvenuti, Curami, "La 'chimera' del Regio Esercito: il carro P.40", Storia Militare, n. 6, 1994, report 63 rounds for the gun and 576 rounds for the two 8 mm machineguns.
Curami was a very reliable source for the technical matters, so I assume his data are the correct ones.