Traveling by Trail/Bus

No, I'd say pay as you go using regional trains which are very cheap. Especially over short distances, Regionale Veloce trains are almost as fast as Freccias.
And buses are even cheaper.

I don't know of a bus pass. You can sometimes save a couple of euro if you buy a roundtrip bus ticket. If you use trains on a weekend, sometimes Trenitalia has a 2-for-1 special.

What cities are you planning on visiting?

Milan, Sirmione, Verona, Venice, Florance, Pisa, Cinque Terra area, Sienna, Sorrento, Amalfi, Capri, Pompeii and Rome.

I am not sure this is even possible by public transportation. 13 stops in 14 days? Might help if you post what you think is feasible. There are lots of people here who can give you estimates of time to travel to these locations.

My title should have read "Train" not "Trail". Sorry. We are planning a few hours in Milan, then a train to Sirmione for the night. A few hours on Sirmione, then train and a few hours in Verona, then train to Venice. So we actually cover 3 towns plus travel to Venice in 2 days. That leaves 12 days to visit the remaining cities. We can remove Cinque Terra or Amalfi if needed. We are flexible.

b kicken

We generally find that it is much easier to consider your plans if you think in terms of how many nights you have than how many days. To get a full day somewhere you need two nights. You arrive one day, spend the night, have a day at the location, spend the night, and move on to the next place the next day.

This list of places

Milan, Sirmione, Verona, Venice, Florance( spelled Florence, Firenze in Italian), Pisa, Cinque Terra area, Sienna (only one "n"), Sorrento, Amalfi, Capri, Pompeii and Rome.

is a good but very busy month or 6 weeks. In 14 days it reminds me of the Griswolds. It probably isn't 14 days, is it? Do you land on day zero, or day 1? Or do you fly on day 1 and land on day 2? And is day 14 the day before you fly home, or is day 14 the actual day you fly home?

13 places in 14 or fewer days means that what you will remember from this trip is what the bus smelled like, the baby crying on the train, the hauls to and from the hotels (or other sleeping accommodation). You won't remember much about the places you paid so much to come and see.

Realistic minimums that I would consider for a first timer with plenty of money and time - in nights - are:

Milan 3
Sirmione dunno, never heard of it
Verona 3
Venice 4 or 5, more if you want to visit Padua and other nearby towns
Florence 4
Pisa day trip from Florence or a night or 2 in Lucca but be sure to add back to Florence
Cinque Terra area 3 or 4
Siena either 3 or day trip from Florence but be sure to add back to Florence
Sorrento 5
Amalfi day trip from Sorrento
Capri on a day with good weather day trip from Sorrento but be sure to add back to Sorrento
Pompeii day trip from Sorrento
Rome 5

So that's 31 nights to 36 nights.

I respectfully suggest something has to go, or just check them off a list and say "I've been there" because you will have done.

Remember that you can always return. Less is often more, much more.

No passes. You only have one trip on which it would be best on a fast train - Venice to Florence.

Everything else is best done on Regionale or Regionale Veloce trains which have very low fares which are the same price up regardless of when you get them, or a few buses. Passes would not pay off in either situation. I don't believe.

how many is "we"?

Milano Centrale to Desenzano del Garda/Sirmione is 9.20 euro regional train, probably best time is 3:35 pm to 4:50 pm.
There to Verona and Venice are regional trains in that price range as well. You'll want to use a fast train from Venice to Florence. Full fare is 45 euro, but advance purchase nonrefundable tickets could be as low as 17 euro.
Sorrento and Pompei are on the local Circumvesuviana Railway. Rail pass won't work, or on the Milan airport train either. Yes something will have to give on this itinerary. But you will find out when you get there and change your plan when reality sets in.

I'll take a whack at this:

Milano, look around after leaving bags at the station, return to retrieve bags, go to Sirmione. At least one night, preferably two.
Venezia: 3 nights, including a daytrip to Verona.
Firenze: 3 nights minimum, including daytrips to Pisa and Siena
Cinque Terre: 2 nights
Sorrento: 4 nights, including daytrips to Capri, Pompeii, Amalfi Coast (you'll be enjoying Sorrento in the evenings)
Roma: 3 nights minimum.

That's 15 nights minimum. Daytrips will cut out some time checking in and out of hotels. You have to cut something, I would suggest Cinque Terre and Sirmione, which would give you 3 fewer nights, bringing you to 12. But I wonder if 12 nights is actually what you have to work with.