What train goes from Camp di Marte train station to S.M. Novella train station in Florence

That is a 7 to 9 minute train ride and they run frequently. This link will show the mid-day schedule (if it works):

https://www.lefrecce.it/B2CWeb/search.do?parameter=searchOutputViewer&cFID=2H4aFcWkfCFB

Note the spelling is Campo di Marte. You need to use the Italian spelling on Trenitalia.

There are many every hour.

Trenitalia runs direct trains run 2 - 3 times per hour between Roma Termini and Florence SMN. If these are your preferred stations, then specify those when searching for a schedule or ticket. Train schedules also available through DB.

To add to my previous post in the other Thread, that's a very easy trip.

When you buy your ticket, specify Firenze Campo di Marte as your departure point and Roma Termini as your arrival point. You'll first take a seven minute train ride from Campo di Marte to Santa Maria Novella (don't forget to validate your ticket prior to boarding), and then transfer to the train to Rome (most likely a Freccia high speed train, travel time about 90 minutes). You can see all the details on either the Trenitalia or bahn.de websites.

There are hourly high speed trains from the Campo di Marte station to Rome (depart 22 minutes after the hour). Why not book one of those trains rather than going to SMN?