I guess I didn't explain myself very well, sorry about that.
Not on every 18th frame, but on the specific frames that I will put on the chose, it could be any number frame I want to put there for example: 2, 29, 72, 115, 188, 201, etc..
For now I just made every specific frame to it's own event, it's just doesn't look as efficient as it could be in one event and a nice expression that I probably not familiar with like they way I used "CHOOSE" to put all the audio I wanted in one expression (I'm aware it's choosing random from the list I put) but it's a good example of one simple expression and I wonder if there is something like that for listing my animation frames, so I can also edit it if I like.
I think your example with the string is something similar to what I look for if there is no built-in expression for that, I'm not sure but I should try it, thanks! :)