six places in 18 days to much?

Each move costs you half a day (time on the train; checking out and in hotels; getting to and from railway station). That leaves you with on average 2,5 days per place. Rome and Venice need more; Florence as well if you want to explore the museums or have a day trip to Siena or Pisa. I would feel happier with one destination less.

Most regular posters have been to the "big" sights and take a more leisurely approach to travel. Me? I like to travel much faster and see more, so you have my vote.

I agree with Bruce on this one. You will have to prioritize what you want to see/do to fit it all in, but you can certainly get a general look around and hit the highlights in that time.

Allocate nights on the ground rather than days.
For Venice I recommend 3 nights
For Florence I also recommend 3 nights, however if you want to use Florence as your base to visit Tuscany and its towns (e.g. Siena, Pisa, Lucca, Pistoia, Arezzo, San Gimignano, etc.) then add one night for each day trip you intend to take from Florence.
For Rome I recommend 4 nights. If you return home from Rome, you should leave Rome for last in your itinerary.
For Amalfi Coast and surrounding (Capri, Sorrento, Pompeii, Naples, etc.) I recommend a minimum of 4 nights.
For Lake Como I recommend 3 nights
Milan can be seen in a day or so, so two nights are sufficient.

Your itinerary (assuming flying into Milan and fly back home from Rome) should be:
Fly into Milan
MILAN
LAKE COMO
VENICE
FLORENCE (TUSCANY)
SORRENTO PENINSULA/AMALFI COAST
ROME

18 nights might be too short for all of the above. Maybe you could reduce Milan to 1 night (after arrival) and Lake Como to 2 nights. Try not to go under 3 nights for Rome. Or you could remove something altogether (like Milan or lake Como). It's up to you.

I believe you can make it work well but would not do exactly 3 nights each.
I prefer to move around as your plan would and really see no problem with it but would make sure you have 4 nights Rome and 4 nights for the Amalfi Coast. Ideally you can fly into Milan and out of Naples.
That means you would need to shave 2 days from elsewhere
Milan is an obvious one and maybe you can even shave 2 from there if you fly into Milan arriving in the AM and just stay 1 night before leaving for Lake Como.
That would be my first choice, just to be able to land, get situated and see the Duomo there and then move on.
My second choice if you go with 2 nights for Milan would be Lake Como or Venice to 2 nights.
Florence you should keep at 3 nights.
I just returned from a similar trip. 7 places in 20 nights and it did not feel rushed because you are vacation for so long.
We only gave 2 nights to Venice, while there is more than enough to do to give it a 3rd night I was a little sick of the insane crowds after 2 nights personally so felt it was enough to see and experience Venice as long as you have no plans to see Murano, Burano, etc... ; Looking back I would have preferred an extra night there but there were multiple other locations I would have rather had another night than Venice so 2 worked for us.
Timing of arrival and departure at each location does factor in and something that I would analyze closely if deciding on which of Lake Como or Venice should drop to 2 nights instead of 3.

To get from Milano to Como (are you staying in the city or in one of the other towns on the lake?) will take less than half a day to Como, add an hour if you are staying in one of the lake towns.

To go to Venezia it's probably easiest to backtrack to Milano Centrale and then take the train. A little more than half a day for travel, checkout, checkin.

Venezia to Firenze Santa Maria Novella is about 3-4 hours, counting vaporetto time to the station, waiting time, travel to Firenze Santa Maria Novella. Then more time to your hotel, depending on where it is.

Firenze to the Amalfi Coast will take up a good part of a day, unless you stay in Sorrento instead (saves 2 hours).

Amalfi Coast to Roma Termini will involve a bus or ferry, a commuter train, and a fast train from Napoli Centrale to Roma Termini, so about 4 hours, less if you stay in Sorrento instead of on the Amalfi Coast.

That leaves you an average of 2.5 days in each place (better to count nights).

You may want to eliminate either the Como section or the Amalfi Coast section, although you could do it all at a quick pace.

But why not fly out of Naples then you are not backtracking going from Florence to Amalfi Coast and then back north to Rome?
Go in order of Milan, Como, Venice, Florence, Rome, Amalfi Coast and then fly out of Naples.

We just got back from Italy two weeks ago, and we stayed in 5 different places over 15 days. It was insane and fast-paced but we loved it!

Venice
Rome (with day trip to Tivoli)
Florence (with 12-hour wine tour through Val d'Orcia)
Umbria (agriturismo in Perugia)
Capri (with day trip to Sorrento/Positano)
Naples (with day trip to Pompeii)

Everyone has a different style of travel, have NO regrets when you leave!