Yes, it is too many cities. Even the most whirlwind of bus tours — where you mostly “see” Europe through the window of a moving bus — would only have 10 stops in 21 days, maybe 11 for your slightly longer than 3 weeks. An average of nearly 3 days per city is the rule of thumb for on-your-own travel, so I’d go for about 9 or 10 for your trip. I’d probably save Spain for your next trip. Use http://reiseauskunft… for train schedules anywhere in Europe to see just how much of your waking time will be spent on trains — add time for checking in and out of hostels, getting to /from train stations, and some time in the station and getting oriented to a new city.

Do you have specific reasons (other than transportation logistics) for Marseilles and Milan?

I’m planning my HS graduation trip around that time as well! I’ll subscribe to this thread to know what you decided. I wanted to visit many cities too, but I ended up planning a kind of U shape around Europe, although visiting Stockholm will be very difficult for me!

Your trip looks amazing! Are you excited to go to Pamplona?

I suggest you work around with that map considering the time you’re going, and maybe 1 day per city is really short time. The map will help you with the feasibility bar to know if you’re planning a “do-able” trip.