Don't trust the planner on the Interrail website. You need to do the calculation of travel day yourself. 

The ferry from Stockholm to Helsinki is not included in the pass so that trip doesn't use a travel day. You get a discount on the normal ticket with the ferry but you should check if there are any special deals for the ferry as that might be cheaper that a normal ticket with Interrail discount. 

You are correct that leaving with a night train that arrives the next morning only uses one travel day. 

For the stretch on the ferry, you don’t need to use a travel day. You get a discount, when you can prove that you’re a pass holder. I think you’ll need to activate your pass, but you don’t need to activate a travel day.

The trip planner is not super reliable, just trust your own counting . 4 Days should suffice if I count correctly. (Unless you would want to take a train to/from the airport, then you’ll need an extra day)

Indeed, a journey started before midnight, arriving the next day (like a night train) counts as one travel day. As soon as you want to take any other train after midnight, you’lld need a new travel day.

Yes, this is correct. The pass has to be valid the day you use the discount on the ferry but no travel day should be used.

i can’t find riga warsaw by train

we will travel through lituania instead of latvia

i confirm all infos on how to include a ferry passage, thanks

after modifications :

we are leaving france and arriving by plane in stockholm on 3/7/23.

our first trip will be going south of sweden

by ferry between sweden and lituania (not incl in pass)

the second between klaipeda and warsaw poland
the third between warsaw and vienna austria
the fourth between vienna and venice italy
the fifth and last for the return to france in modane

why this projet doesn’t fit with 5 days pass ?

There are currently no trains from Latvia to Lithuania, so you would need a bus for at least part of that journey.

From where in Sweden do you take the ferry to Latvia? 

@AnnaB

i found a ferry from Karlshamn Sweden to Klaipeda Lithuania

to latvia, there are ferries between Nynashamn and Ventspils, since the stockholm - riga doesn’t seem to sail at the moment

well, i rephrase my question, theoretically :

i’m living in france F

in july i fly to A

first train trip from A to B

2nd from B to C

3rd from C to D

4th from D to E

5th and last from E back to france F

which global pass do I have to purchase ?

That's 5 travel days. But check timetables first, before you buy anything. E.g. on bahn.com.