Sorry for taking so long,
different projects pushed their way into my work life.
Thanks for your reply 
I understand that multiple sounds at once should not be problematic, for some reason they are. Could it be the general quality of my wav files? They are 16 bit, 44.1khz format. If I play them outside of the project, one sound at a time, I encounter no problems. When testing inside the editor (and in standalone) I encounter artifacts, mostly crackling noises. They only appear when I hit one piano key (only the same key) in rapid succession (about twice in one second). I believe it has something to do with starting the same cue again, while it is still active, but my guess is completely blind.
For now I have all the piano keys as a blueprint with an audio component and an overlap box, and all the sounds in a sound cue each.
The audio component is the corresponding sound cue and plays through a do-once gate whenever I overlap the box.
Upon ending the overlap the sound component does a fade out and the do-once gate for playing the sound component is being resetted.
I have done this without the fade out, with delay after the fade out and with a complete stop. If the delay is long enough, the artifacts disappear, but I obviously cannot play the key “fast”. Wihout any fade out or with a stop node I get the crackling artifacts. The sudden stop+restart seems to be a problem and I don’t know if this is a usual problem, that can be avoided.