My question is quite simple : ok, you can create objects on the fly - but can you destroy them, then, to prevent any memory problem ?
No. You have to reuse created objects if you want to keep creating them forever.
There’s some internal reason for this which I don’t remember offhand.
any idea of how many objects a game can have before having memory problems ?
It’s hard to say, because the problems will start as gradual game slowdown. In a Glulx game you could arrange to make thousands of objects available, but the game will become unplayable at some point. How far you push it is kind of up to you.