which website for train from Utretch -Ghent, and so on

1 & 2) Here is the universal travel planner for Netherlands.
https://9292.nl/en

3) Netherlands Railway International
https://www.nsinternational.nl/en
Or Belgian Railway International.
https://www.b-europe.com/EN

4 & 5) Belgian Railway Domestic.
http://www.belgianrail.be/en/

6) What hotel do you have booked?

There are 3 basic ways to get into Venice. Water taxi is expensive, like 110 EUR for up to 4 people, will take you right to your hotel if it is on the water, or as close as the boat can get it if not. From the airport dock which is about 500 meters from the airport with a moving walkway part of the way.
https://www.motoscafivenezia.it/

Alilaguna bus boat runs a regular schedule for about 15 EUR pp. Takes while depending on where on the route your hotel is. From the airport dock which is about 500 meters from the airport with a moving walkway part of the way.
https://www.alilaguna.it/en

No 5 bus to the Piazzale Roma bus depot, then vaporetto bus boat to the dock closest to your hotel. Cost is 14 EUR for the combined bus + vaporetto ticket. Or, if you get a multi-day vaporetto transport pass, airport bus can be added for 6 EUR at the time of purchase. Bus goes from the airport door.
http://actv.avmspa.it/en/content/prices

In addition:
1. The Amsterdam & Region Travel Ticket is not valid for Utrecht. Buy those tickets (Schiphol-Utrecht, Utrecht-Amsterdam v.v.) at the railway station (at the window or from a machine). Use website 9292.nl only for consulting schedules.
2. The tickets for Utrecht-Gent can be bought at the railway station in Utrecht. There is no reason for advance buying (no discount, no reservation).
3. When arriving in Gent with your luggage, take a taxi to your accommodation. Should be around EUR 10.
4. For getting to and from the railway station in Gent for your day trips, you can take the tram from the city centre. This map shows the lines and the stops. Tickets can be bought from the machine at the tram stop. Have enough loose euro coins at hand.
5. Like in the Netherlands, buy your tickets at the railway station.

Sam,

I would want to book hotel in venice, florence, and milan soon as hotels price go up and limited choice. I have few questions under this link and waiting for response.

https://community.ricksteves.com/travel-forum/italy/oct-2018-trip-itinerary-needs-suggestion-thanks

I am hoping to get some idea regarding how many N to stay in my question. Would you able to contribute some experience ?

Looking at that post, I suggest you buy the Brussels Airlines "Light & Relax" now. Then you just have to plan a 10 night trip to north Italy. So now just decide how many times you want to pull up stakes and move.

I am sorting out the train/railway route these days since my trip is 2 months away.

I read through the comment again, and is it correct that I would use the b-europe.com website for these locations below, i.e. when transfer from Utretch-Amsterdam (vice-versa), Utretch- Ghent, Ghent-Bruges, and G-Antwerp ( 4N in Ghent, then day trip between cities), and G- Brussels train, then from train station to BRU airport.

Or Beligum railway international is applied to Utretch-Ghent ?

Beligum railway domestic is Ghent-Bruges, and Antwerp ?

Can I buy 10 ticket pass to be used for both Belgium international and domestic , or I should just buy each time when I ride the railway because I won't able to save $ by purchasing the 10 ticket pass.

Please clarify.

Thanks.

is it correct that I would use the b-europe.com website
Use them only for the trip from Utrecht (mind the spelling) to Gent. You can buy those tickets at Utrecht station, but that might come with a handling fee. Take the Intercity service instead of the Thalys. Tickets for the Intercity service are valid on all trains that day, while the Thalys comes with a mandatory reservation for a specific train.
All other tickets are best bought at the station, either from a machine or at the window. There is no advantage in buying them in advance (with the NS for the Netherlands, and NMBS/SNCB for Belgium), and all these trains - both in the Netherlands and in Belgium - come without reservation. If you miss a train, you take the next. They run amazingly frequently in these parts.