There are some reservation that have to be printed (on the E - Reservations its written when you have to print)

the paper reservations that will be shipped are like that because they need a specific International Rail travel Format (CIV Standard)

Thanks for the advice!

Another thing: Is it correct that I can't make more than 1 reservation at the same time on the Interrail site?

I'm trying to make 2 seat reservations for the TGV Lyon - Bruxelles, but Interrail only gives an option to book for 1 person.

Yeah for that you have to add a traveler to your trip to book for 2 persons :)