You need reservations for all long distance trains. Availability is usually good, even on the day.

Have a look here:

https://www.eurail.com/en/plan-your-trip/trip-ideas/top-destinations/italy-train

https://www.seat61.com/train-travel-in-italy.htm

https://www.seat61.com/interrail-and-eurail-reservations.htm#Italy

All Highspeed trains have mandatory reservations, all long distance Intercity have too.
Freccia Argenta and Bianca are 10€, Freccia Rossa 13€ and IC 3€.
To be booked on tickets.oebb.at , add Interrail discount and click “One Way tickets” NOT Seat only.
Regionale and Regionale Veloce don’t need reservations