Travel plans: How many countries with a 7days Pass?

Within one week, that's a bit much, I'd say, especially if you want to have time to look around.

If you mean using 7 travel days over a longer period, that should work.

Make a planning with timetables to get from one place to another and then see how much you can or want to fit in.

IF you plan sharply (and have luck with getting it!) and you take a normal discount ferry then you could do it from UK on 2 traveldays this way-which I really think should not be too much of a challenge to find out for a student used to do some research.

Coming also back this way needs an extra day-due to how the overnight trains fit in the scheme

And its all as rian already tells: decide which stops en-route you wish to do. For the 2 days its only Nice-or close by. Those IT ferryports (Bari, Brindisi) are not really worth more as the few hrs anyone sits there waiting for the boat.