Yes, on any travel day, you can take as many trains as you like.

The time of departure counts, so if you take a night train, it doesn't use a second travel day.

Bear in mind that you have to pay a reservation fee on direct ICE or TGV between Germany and France, and on the Eurostar to Amsterdam. Details here:

https://www.seat61.com/interrail-and-eurail-reservations.htm

I just want to mention the Interrail limitation : you can only travel on 2 pass days in your own country. No limitation with Eurail.

This means that you can't do all these journeys with a pass. You'll have to buy a ticket for some of the journeys.

Stuttgart - Paris : you can go until Kehl with the Deutschlandticket but it obviously takes longer. There is also a special pass fare available on international ICE/TGVs but I don't know the price. Better ask at a counter and show this to staff in case : https://public-pf.b-europe.com/en/Producten/Details?id=13daa928-b39f-4e69-a479-66eb71463b54

Same thing for Salzburg - Stuttgart : doable but longer with Deutschlandticket. Also possible to go via Zurich (pass valid until Schaffhausen, then D-Ticket valid on those IC trains) if there are no engineering works. For example :

- RJX Wien Hbf - Zürich HB 07:28 - 15:20 (or later)

- IC Zürich HB - Stuttgart Hbf 15:34 - 18:43 or 16:34 - 19:32

What would be your travel dates ? As ralderton said be careful with mandatory reservations around Christmas & New Year (aka book ASAP).

Questions welcome.

Thank you for this detailed information, but we don’t use Interrail because my friend is not a European Citizen. We want to do the Global pass with Eurorail. 

If you’re German living in Germany, you’d need to buy the Interrail pass. Your Kenyan friend could buy the Eurail pass. (If you’re a non-European resident in Europe, or a German living outside Europe, you could also buy the Eurail pass.)

https://www.eurail.com/en/help/interested-in-eurailing/do-i-need-a-eurail-or-an-interrail-pass

They both have a “Global Pass” version.

You can of course still travel together. But if you have to buy the Interrail pass, you would be limited to two travel days in Germany.

That doesn’t make any sense. I’m German and I don’t want to travel within Germany, I want to travel from Stuttgart-Paris

Paris-Amsterdam, 

Amsterdam-Berlin,

Berlin-Vienna

Vienna-Stuttgart.

So its best do go to the DB counter and ask, because it’s a little confusing. :) 

Thank you for your help.

Well you cannot go to Paris without travelling in Germany first…

There are 4 travel days in which you'll be in Germany. You can have max 2 (at any point of the trip) so on 2 days you'll have to buy tickets for the leg within Germany.

These inbound/outbound journeys were designed to go to the border to start your trip and come back in the end. Until a few years ago you had to pay your own ticket to the border, no matter what !

Any travel to, from, or through Germany - it counts as one of your travel days. I don’t make the rules!

It is quite unfair for Germans, because so many journeys must pass through your own country.

So you could maybe pay cash for some of your journeys. Or reroute as much of the trip as possible through Switzerland/ Austria/ Czechia/ Poland.

If you have the Deutschlandticket, you could perhaps use that to reach the border, but it’s not valid on IC, ICE, EC or Eurostar.

If you can move your itinerary to travel in a different order, you can do it with only two travel days in Germany, and a little extra ticket.

Stuttgart-Berlin Home Travel Day 1]
Berlin-Prague-Vienna pay your own ticket to the DE-CZ border]
Vienna-Basel/Geneva-Paris
Paris-Amsterdam
Amsterdam-Stuttgart Home Travel Day 2]