Trophy unlocked with only 12 c64 demos for me.
Since they are i believe mostly randomly generated, there is no real guide for them.
I had:
2nd reality
Triage
Triage 2
Triage 3
Triage 4
Triage 5 (only with with number as roman numeral)
Our darkness
Fallout
Electric
Type o negative
Exocute overdrive
Industrial revolution
To “start” the demos, press/hold triangle select the demo and press x on the c64 in the bedroom to insert the demo, then make sure to press the keyboard next to it. You should see a small red light on the top left of the keyboard turn on and the computer screen just goes black. Take the demo out replace with the next, repeat.
Aside from the c64 demos theres also a few cassette tapes, which may or may not be needed.
They are:
Northpolyptica “herz,lass nach”?
Nightwatch and the witch
Rock classics vis extra v90
To use the cassettes go to the boombox in the other bedroom and put the cassette in the same way as the demos, then press the actual play button on the boombox.